diff --git a/src/Schematron/EvaluableExpression.cs b/src/Schematron/EvaluableExpression.cs
index 971aff0..f626d98 100644
--- a/src/Schematron/EvaluableExpression.cs
+++ b/src/Schematron/EvaluableExpression.cs
@@ -14,8 +14,8 @@ namespace Schematron;
 /// <progress amount="100" />
 public abstract class EvaluableExpression
 {
-    string xpath = null!;
-    XPathExpression expr = null!;
+    string? xpath;
+    XPathExpression? expr;
     XmlNamespaceManager? ns;
 
     /// <summary>
@@ -39,7 +39,7 @@ protected void InitializeExpression(string xpathExpression)
         expr = Config.DefaultNavigator.Compile(xpathExpression);
         ret = expr.ReturnType;
 
-        if (ns != null)
+        if (ns is not null)
             expr.SetContext(ns);
     }
 
@@ -48,10 +48,10 @@ protected void InitializeExpression(string xpathExpression)
     /// A clone of the expression is always returned, because the compiled
     /// expression is not thread-safe for evaluation.
     /// </remarks>
-    public XPathExpression CompiledExpression => expr != null ? expr.Clone() : null!;
+    public XPathExpression? CompiledExpression => expr?.Clone();
 
     /// <summary>Contains the string version of the expression.</summary>
-    public string Expression => xpath;
+    public string Expression => xpath ?? string.Empty;
 
     /// <summary>Contains the string version of the expression.</summary>
     public XPathResultType ReturnType => ret;
@@ -62,7 +62,7 @@ protected void InitializeExpression(string xpathExpression)
     /// <summary>Sets the manager to use to resolve expression namespaces.</summary>
     public void SetContext(XmlNamespaceManager nsManager)
     {
-        if (expr != null)
+        if (expr is not null)
         {
             // When the expression contains variable references ($name), .NET requires an
             // XsltContext (not just XmlNamespaceManager). Use a load-time stub that satisfies
diff --git a/src/Schematron/EvaluationContextBase.cs b/src/Schematron/EvaluationContextBase.cs
index a00ae75..5c5f380 100644
--- a/src/Schematron/EvaluationContextBase.cs
+++ b/src/Schematron/EvaluationContextBase.cs
@@ -37,7 +37,7 @@ public abstract class EvaluationContextBase
     /// strategy for matching nodes is initialized, depending on the specific
     /// implementation of the <see cref="XPathNavigator"/> in use.
     /// </remarks>
-    protected IMatchedNodes? Matched { get; set; }
+    protected IMatchedNodes Matched { get; set; } = NullMatchedNodes.Instance;
 
     /// <summary>Gets or sets the class to use to format messages.</summary>
     /// <remarks>
@@ -66,7 +66,7 @@ public abstract class EvaluationContextBase
     public string Phase { get; set; } = string.Empty;
 
     /// <summary>Gets or sets the schema to use for the validation.</summary>
-    public Schema? Schema { get; set; }
+    public Schema Schema { get; set; } = Schema.Empty;
 
     /// <remarks>
     /// When this property is set, the appropriate <see cref="IMatchedNodes"/>
@@ -74,7 +74,7 @@ public abstract class EvaluationContextBase
     /// </remarks>
     public XPathNavigator Source
     {
-        get => source!;
+        get => source ?? throw new InvalidOperationException("Source has not been set.");
         set
         {
             source = value;
@@ -108,6 +108,9 @@ public XPathNavigator Source
     /// <remarks>
     /// By default, it clears the <see cref="Messages"/> and sets <see cref="HasErrors"/> to false.
     /// </remarks>
-    protected void Reset() => Messages = new StringBuilder();
+    protected void Reset()
+    {
+        Messages.Clear();
+        HasErrors = false;
+    }
 }
-
diff --git a/src/Schematron/Formatters/FormatterBase.cs b/src/Schematron/Formatters/FormatterBase.cs
index ee7502a..7243b85 100644
--- a/src/Schematron/Formatters/FormatterBase.cs
+++ b/src/Schematron/Formatters/FormatterBase.cs
@@ -104,7 +104,7 @@ protected static StringBuilder FormatMessage(Test source, XPathNavigator context
             sb.Append(msg[offset..name.Index]);
 
             // Does the name element have a path attribute?
-            if (nameExpr != null)
+            if (nameExpr is not null)
             {
                 SetExpressionContext(nameExpr, source, ambientCtx);
 
@@ -122,11 +122,11 @@ protected static StringBuilder FormatMessage(Test source, XPathNavigator context
                     result = context.Evaluate(nameExpr) as string;
                 }
 
-                if (result != null)
+                if (result is not null)
                     sb.Append(result);
             }
             // Does the value-of element have a select attribute?
-            else if (selectExpr != null)
+            else if (selectExpr is not null)
             {
                 SetExpressionContext(selectExpr, source, ambientCtx);
 
@@ -143,7 +143,7 @@ protected static StringBuilder FormatMessage(Test source, XPathNavigator context
                     result = context.Evaluate(selectExpr) as string;
                 }
 
-                if (result != null)
+                if (result is not null)
                     sb.Append(result);
             }
             // If there is no path or select expression, there is an empty <name> element.
@@ -159,14 +159,14 @@ protected static StringBuilder FormatMessage(Test source, XPathNavigator context
 
     static void SetExpressionContext(XPathExpression expr, Test source, SchematronXsltContext? ambientCtx)
     {
-        if (ambientCtx != null)
+        if (ambientCtx is not null)
         {
             try { expr.SetContext(ambientCtx); return; }
             catch (System.Xml.XPath.XPathException) { /* fall through */ }
         }
 
         var ns = source.GetContext();
-        if (ns == null) return;
+        if (ns is null) return;
         try { expr.SetContext(ns); }
         catch (System.Xml.XPath.XPathException)
         {
diff --git a/src/Schematron/IMatchedNodes.cs b/src/Schematron/IMatchedNodes.cs
index 08e4a93..c1e425d 100644
--- a/src/Schematron/IMatchedNodes.cs
+++ b/src/Schematron/IMatchedNodes.cs
@@ -35,3 +35,13 @@ public interface IMatchedNodes
     void Clear();
 }
 
+/// <summary>No-op sentinel used as a default before a real matching strategy is selected.</summary>
+class NullMatchedNodes : IMatchedNodes
+{
+    public static NullMatchedNodes Instance { get; } = new();
+
+    public bool IsMatched(XPathNavigator node) => false;
+    public void AddMatched(XPathNavigator node) { }
+    public void Clear() { }
+}
+
diff --git a/src/Schematron/Schema.cs b/src/Schematron/Schema.cs
index b0f0ae4..1a21869 100644
--- a/src/Schematron/Schema.cs
+++ b/src/Schematron/Schema.cs
@@ -23,6 +23,9 @@ public class Schema
     /// <summary>The default Schematron namespace. Kept for backward compatibility; prefer <see cref="IsoNamespace"/>.</summary>
     public const string Namespace = LegacyNamespace;
 
+    /// <summary>A shared empty schema instance used as a default sentinel.</summary>
+    public static Schema Empty { get; } = new();
+
     /// <summary>Returns <see langword="true"/> if <paramref name="uri"/> is a recognized Schematron namespace URI.</summary>
     public static bool IsSchematronNamespace(string? uri) => uri == IsoNamespace || uri == LegacyNamespace;
 
@@ -109,6 +112,6 @@ public void Load(XmlReader schema)
     public bool IsLibrary { get; set; }
 
     /// <summary />
-    public XmlNamespaceManager NsManager { get; set; } = null!;
+    public XmlNamespaceManager NsManager { get; set; } = new(new NameTable());
 }
 
diff --git a/src/Schematron/SchemaLoader.cs b/src/Schematron/SchemaLoader.cs
index 45998b8..8f98ab7 100644
--- a/src/Schematron/SchemaLoader.cs
+++ b/src/Schematron/SchemaLoader.cs
@@ -1,4 +1,5 @@
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using System.Xml;
 using System.Xml.XPath;
 
@@ -14,31 +15,37 @@ namespace Schematron;
 public class SchemaLoader(Schema schema)
 {
     XPathNavigator filenav = null!;
-    Hashtable? abstracts = null;
+    Hashtable? abstracts;
 
     // Detected Schematron namespace and the namespace manager derived from the source document.
-    string schNs = null!;
-    XmlNamespaceManager mgr = null!;
+    string? schNs;
+    XmlNamespaceManager? mgr;
 
     // Instance-level XPath expressions compiled against the detected namespace.
-    XPathExpression exprSchema = null!;
-    XPathExpression exprEmbeddedSchema = null!;
-    XPathExpression exprPhase = null!;
-    XPathExpression exprPattern = null!;
-    XPathExpression exprAbstractRule = null!;
-    XPathExpression exprConcreteRule = null!;
-    XPathExpression exprRuleExtends = null!;
-    XPathExpression exprAssert = null!;
-    XPathExpression exprReport = null!;
-    XPathExpression exprLet = null!;
-    XPathExpression exprDiagnostic = null!;
-    XPathExpression exprParam = null!;
-    XPathExpression exprLibrary = null!;
-    XPathExpression exprRulesContainer = null!;
-    XPathExpression exprGroup = null!;
+    XPathExpression? exprSchema;
+    XPathExpression? exprEmbeddedSchema;
+    XPathExpression? exprPhase;
+    XPathExpression? exprPattern;
+    XPathExpression? exprAbstractRule;
+    XPathExpression? exprConcreteRule;
+    XPathExpression? exprRuleExtends;
+    XPathExpression? exprAssert;
+    XPathExpression? exprReport;
+    XPathExpression? exprLet;
+    XPathExpression? exprDiagnostic;
+    XPathExpression? exprParam;
+    XPathExpression? exprLibrary;
+    XPathExpression? exprRulesContainer;
+    XPathExpression? exprGroup;
 
     /// <summary />
     /// <param name="source"></param>
+    [MemberNotNull(nameof(schNs), nameof(mgr),
+        nameof(exprSchema), nameof(exprEmbeddedSchema), nameof(exprPhase),
+        nameof(exprPattern), nameof(exprAbstractRule), nameof(exprConcreteRule),
+        nameof(exprRuleExtends), nameof(exprAssert), nameof(exprReport),
+        nameof(exprLet), nameof(exprDiagnostic), nameof(exprParam),
+        nameof(exprLibrary), nameof(exprRulesContainer), nameof(exprGroup))]
     public virtual void LoadSchema(XPathNavigator source)
     {
         schema.NsManager = new XmlNamespaceManager(source.NameTable);
@@ -84,6 +91,12 @@ public virtual void LoadSchema(XPathNavigator source)
     /// Detects the Schematron namespace used in <paramref name="source"/> and compiles all
     /// instance-level XPath expressions against that namespace.
     /// </summary>
+    [MemberNotNull(nameof(schNs), nameof(mgr),
+        nameof(exprSchema), nameof(exprEmbeddedSchema), nameof(exprPhase),
+        nameof(exprPattern), nameof(exprAbstractRule), nameof(exprConcreteRule),
+        nameof(exprRuleExtends), nameof(exprAssert), nameof(exprReport),
+        nameof(exprLet), nameof(exprDiagnostic), nameof(exprParam),
+        nameof(exprLibrary), nameof(exprRulesContainer), nameof(exprGroup))]
     void DetectAndBuildExpressions(XPathNavigator source)
     {
         schNs = DetectSchematronNamespace(source);
@@ -428,7 +441,7 @@ void LoadExtends(Rule rule, XPathNavigator context)
         while (extends.MoveNext())
         {
             var ruleName = extends.Current.GetAttribute("rule", string.Empty);
-            if (abstracts != null && abstracts.ContainsKey(ruleName))
+            if (abstracts?.ContainsKey(ruleName) == true)
                 rule.Extend((Rule)abstracts[ruleName]!);
             else
                 throw new BadSchemaException("The abstract rule with id=\"" + ruleName + "\" is used but not defined.");
diff --git a/src/Schematron/SyncEvaluationContext.cs b/src/Schematron/SyncEvaluationContext.cs
index 3f1d067..52e3d41 100644
--- a/src/Schematron/SyncEvaluationContext.cs
+++ b/src/Schematron/SyncEvaluationContext.cs
@@ -27,7 +27,7 @@ public override void Start()
         Reset();
 
         // Is there something to evaluate at all?
-        if (Schema is null || Schema.Patterns.Count == 0)
+        if (Schema.Patterns.Count == 0)
             return;
 
         // If no phase was received, try the default phase defined for the schema.
@@ -35,7 +35,7 @@ public override void Start()
         if (Phase == string.Empty)
             Phase = Schema.DefaultPhase is { Length: > 0 } phase ? phase : Schematron.Phase.All;
 
-        if (Phase != Schematron.Phase.All && Schema.Phases[Phase] == null)
+        if (Phase != Schematron.Phase.All && Schema.Phases[Phase] is null)
             throw new ArgumentException("The specified Phase isn't defined for the current schema.");
 
         if (Evaluate(Schema.Phases[Phase], Messages))
@@ -118,13 +118,14 @@ bool Evaluate(Pattern pattern, StringBuilder output)
         // Reset matched nodes, as across patters, nodes can be 
         // evaluated more than once.
         Matched.Clear();
-        var isGroup = pattern is Group;
 
         foreach (var rule in pattern.Rules)
         {
             // For groups (ISO Schematron 2025), each rule evaluates independently.
-            if (isGroup) Matched.Clear();
-            if (Evaluate(rule, sb, BuildLets(pattern.Lets))) failed = true;
+            if (pattern is Group)
+                Matched.Clear();
+            if (Evaluate(rule, sb, BuildLets(pattern.Lets)))
+                failed = true;
         }
         if (failed)
         {
@@ -176,7 +177,8 @@ bool Evaluate(Rule rule, StringBuilder output, Dictionary<string, string>? patte
         var failed = false;
         var sb = new StringBuilder();
         Source.MoveToRoot();
-        var nodes = Source.Select(rule.CompiledExpression);
+        // Non-abstract rules always have a compiled expression (guarded by IsAbstract check above).
+        var nodes = Source.Select(rule.CompiledExpression!);
         var evaluables = new ArrayList(nodes.Count);
 
         // The navigator doesn't contain line info
@@ -253,7 +255,7 @@ bool Evaluate(Rule rule, StringBuilder output, Dictionary<string, string>? patte
     bool EvaluateAssert(Assert assert, XPathNavigator context, StringBuilder output,
         Dictionary<string, string>? patternLets = null, LetCollection? ruleLets = null)
     {
-        var expr = PrepareExpression(assert.CompiledExpression, context, patternLets, ruleLets, out var xsltCtx);
+        var expr = PrepareExpression(assert.CompiledExpression!, context, patternLets, ruleLets, out var xsltCtx);
         var eval = context.Evaluate(expr);
         var result = true;
 
@@ -293,7 +295,7 @@ bool EvaluateAssert(Assert assert, XPathNavigator context, StringBuilder output,
     bool EvaluateReport(Report report, XPathNavigator context, StringBuilder output,
         Dictionary<string, string>? patternLets = null, LetCollection? ruleLets = null)
     {
-        var expr = PrepareExpression(report.CompiledExpression, context, patternLets, ruleLets, out var xsltCtx);
+        var expr = PrepareExpression(report.CompiledExpression!, context, patternLets, ruleLets, out var xsltCtx);
         var eval = context.Evaluate(expr);
         var result = false;
 
@@ -322,13 +324,18 @@ bool EvaluateReport(Report report, XPathNavigator context, StringBuilder output,
 
     Dictionary<string, string> BuildLets(LetCollection? extraLets = null)
     {
-        var d = new Dictionary<string, string>(StringComparer.Ordinal);
+        var result = new Dictionary<string, string>(StringComparer.Ordinal);
+
         foreach (var let in Schema.Lets)
-            if (let.Value is not null) d[let.Name] = let.Value;
-        if (extraLets != null)
+            if (let.Value is not null)
+                result[let.Name] = let.Value;
+
+        if (extraLets is not null)
             foreach (var let in extraLets)
-                if (let.Value is not null) d[let.Name] = let.Value;
-        return d;
+                if (let.Value is not null)
+                    result[let.Name] = let.Value;
+
+        return result;
     }
 
     // Prepares an XPathExpression to be evaluated with variable support.
@@ -341,14 +348,20 @@ XPathExpression PrepareExpression(
         out SchematronXsltContext? xsltCtx)
     {
         var hasVars = (Schema.Lets.Count + (patternLets?.Count ?? 0) + (ruleLets?.Count ?? 0)) > 0;
-        if (!hasVars) { xsltCtx = null; return expr; }
+        if (!hasVars)
+        {
+            xsltCtx = null;
+            return expr;
+        }
 
         var vars = new Dictionary<string, string>(StringComparer.Ordinal);
         foreach (var let in Schema.Lets)
             if (let.Value is not null) vars[let.Name] = let.Value;
-        if (patternLets != null)
+
+        if (patternLets is not null)
             foreach (var kv in patternLets) vars[kv.Key] = kv.Value;
-        if (ruleLets != null)
+
+        if (ruleLets is not null)
             foreach (var let in ruleLets)
                 if (let.Value is not null) vars[let.Name] = let.Value;
 
diff --git a/src/Schematron/Validator.cs b/src/Schematron/Validator.cs
index cc5b5bd..90cf9b5 100644
--- a/src/Schematron/Validator.cs
+++ b/src/Schematron/Validator.cs
@@ -20,7 +20,7 @@ public class Validator
     readonly SchemaCollection schematrons = [];
     NavigableType navtype = NavigableType.XPathDocument;
 
-    StringBuilder? errors;
+    StringBuilder errors = new();
     bool haserrors;
 
     /// <summary>
@@ -191,7 +191,7 @@ public void AddSchema(XmlReader reader)
         if (wxs)
         {
             haserrors = false;
-            errors = new StringBuilder();
+            errors.Clear();
 
             var xs = XmlSchema.Read(new XmlTextReader(r, reader.NameTable), new ValidationEventHandler(OnValidation));
 
@@ -228,11 +228,11 @@ public void AddSchema(XmlReader reader)
         var sch = new Schema();
         sch.Load(nav);
         schematrons.Add(sch);
-        errors = null;
+        errors.Clear();
     }
 
 
-    #region WORK IN PROGRESS :: The need the for the signature AddSchema(string targetNamespace, string schemaUri) comes from resolving imported (schemaLocation hinted) partial schemas
+    #region WORK IN PROGRESS:: The need the for the signature AddSchema(string targetNamespace, string schemaUri) comes from resolving imported (schemaLocation hinted) partial schemas
 
     bool TryAddXmlSchema(
         string targetNamespace,
@@ -258,7 +258,7 @@ bool TryAddXmlSchema(
         if (!IsStandardSchema(namespaceUri))
             return false;
 
-        errors ??= new StringBuilder();
+        errors.Clear();
 
         var set = new XmlSchemaSet
         {
@@ -307,7 +307,7 @@ public void AddSchema(string targetNamespace, string schemaUri)
         sch.Load(nav);
 
         schematrons.Add(sch);
-        errors = null;
+        errors.Clear();
     }
 
     #endregion
@@ -334,7 +334,7 @@ public void AddSchema(string targetNamespace, string schemaUri)
     /// </exception>
     public void ValidateSchematron(XPathNavigator file)
     {
-        errors = new StringBuilder();
+        errors.Clear();
         Context.Source = file;
 
         foreach (var sch in schematrons)
@@ -394,12 +394,12 @@ public IXPathNavigable Validate(string uri)
     /// <returns>The loaded <see cref="IXPathNavigable"/> instance.</returns>
     public IXPathNavigable Validate(XmlReader reader)
     {
-        errors = new StringBuilder();
+        errors.Clear();
 
         var hasxml = false;
-        StringBuilder? xmlerrors = null;
+        string? xmlErrorText = null;
         var hassch = false;
-        StringBuilder? scherrors = null;
+        string? schErrorText = null;
 
         var settings = new XmlReaderSettings
         {
@@ -439,14 +439,14 @@ public IXPathNavigable Validate(XmlReader reader)
             Context.Formatter.Format(r.Settings.Schemas, errors);
             Context.Formatter.Format(r, errors);
             hasxml = true;
-            xmlerrors = errors;
+            xmlErrorText = errors.ToString();
         }
 
         Context.Source = nav;
 
         // Reset shared variables
         haserrors = false;
-        errors = new StringBuilder();
+        errors.Clear();
 
         foreach (var sch in schematrons)
         {
@@ -462,12 +462,14 @@ public IXPathNavigable Validate(XmlReader reader)
         {
             Context.Formatter.Format(schematrons, errors);
             hassch = true;
-            scherrors = errors;
+            schErrorText = errors.ToString();
         }
 
-        errors = new StringBuilder();
-        if (hasxml) errors.Append(xmlerrors!.ToString());
-        if (hassch) errors.Append(scherrors!.ToString());
+        errors.Clear();
+        if (hasxml)
+            errors.Append(xmlErrorText);
+        if (hassch)
+            errors.Append(schErrorText);
 
         if (hasxml || hassch)
         {
@@ -487,7 +489,7 @@ void PerformValidation(Schema schema)
     void OnValidation(object sender, ValidationEventArgs e)
     {
         haserrors = true;
-        Context.Formatter.Format(e, errors!);
+        Context.Formatter.Format(e, errors);
     }
 }