use char instead of string for joins

TUnit.Analyzers.CodeFixers/Base/AsyncMethodSignatureRewriter.cs

@@ -84,7 +84,7 @@ private static string GetMethodKey(MethodDeclarationSyntax node)
         // Build a key from class name, method name, and parameter types
         var className = node.Ancestors().OfType<TypeDeclarationSyntax>().FirstOrDefault()?.Identifier.Text ?? "";
         var methodName = node.Identifier.Text;
-        var parameters = string.Join(",", node.ParameterList.Parameters.Select(p => p.Type?.ToString() ?? ""));
+        var parameters = string.Join(',', node.ParameterList.Parameters.Select(p => p.Type?.ToString() ?? ""));
         return $"{className}.{methodName}({parameters})";
     }
 

TUnit.Assertions.SourceGenerator/Generators/AssertionExtensionGenerator.cs

@@ -360,8 +360,8 @@
         // The extension method extends IAssertionSource<T> where T is the type argument
         // from the Assertion<T> base class.
         string sourceType;
         string genericTypeParam = null;
         string genericConstraint = null;
 
         if (isNullableOverload)
         {
@@ -369,8 +369,8 @@
             // because NRT annotations are erased at runtime - they're the same type to the CLR.
             // Instead, just use the nullable version and accept both nullable and non-nullable sources.
             sourceType = $"IAssertionSource<{typeParam.ToDisplayString()}>";
             genericTypeParam = null;
             genericConstraint = null;
         }
         else if (typeParam is ITypeParameterSymbol baseTypeParam)
         {
@@ -422,7 +422,7 @@
         if (allConstraints.Count > 0)
         {
             sourceBuilder.AppendLine();
-            sourceBuilder.Append($"        {string.Join(" ", allConstraints)}");
+            sourceBuilder.Append($"        {string.Join(' ', allConstraints)}");
         }
 
         sourceBuilder.AppendLine();

TUnit.Assertions.SourceGenerator/Generators/AssertionMethodGenerator.cs

@@ -1167,7 +1167,7 @@ private static void GenerateMethod(StringBuilder sourceBuilder, string targetTyp
         {
             // Add generic constraints from the method
             sourceBuilder.AppendLine();
-            sourceBuilder.Append($"        {string.Join(" ", genericConstraints)}");
+            sourceBuilder.Append($"        {string.Join(' ', genericConstraints)}");
         }
 
         sourceBuilder.AppendLine();

TUnit.Assertions.SourceGenerator/Generators/MethodAssertionGenerator.cs

@@ -20,7 +20,7 @@
     public static string BuildAssertion = "MethodAssertionData";
 
     private static readonly DiagnosticDescriptor MethodMustBeStaticRule = new DiagnosticDescriptor(
         id: "TUNITGEN001",
         title: "Method must be static",
         messageFormat: "Method '{0}' decorated with [GenerateAssertion] must be static",
         category: "TUnit.Assertions.SourceGenerator",
@@ -29,7 +29,7 @@
         description: "Methods decorated with [GenerateAssertion] must be static to be used in generated assertions.");
 
     private static readonly DiagnosticDescriptor MethodMustHaveParametersRule = new DiagnosticDescriptor(
         id: "TUNITGEN002",
         title: "Method must have at least one parameter",
         messageFormat: "Method '{0}' decorated with [GenerateAssertion] must have at least one parameter (the value to assert)",
         category: "TUnit.Assertions.SourceGenerator",
@@ -38,9 +38,9 @@
         description: "Methods decorated with [GenerateAssertion] must have at least one parameter representing the value being asserted.");
 
     private static readonly DiagnosticDescriptor UnsupportedReturnTypeRule = new DiagnosticDescriptor(
         id: "TUNITGEN003",
         title: "Unsupported return type",
         messageFormat: "Method '{0}' decorated with [GenerateAssertion] has unsupported return type '{1}'. Supported types are: bool, AssertionResult, AssertionResult<T>, Task<bool>, Task<AssertionResult>, Task<AssertionResult<T>>",
         category: "TUnit.Assertions.SourceGenerator",
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true,
@@ -49,7 +49,7 @@
     private static readonly DiagnosticDescriptor RefStructRequiresInliningRule = new DiagnosticDescriptor(
         id: "TUNITGEN004",
         title: "Ref struct parameter requires method body inlining",
         messageFormat: "Method '{0}' has ref struct parameter '{1}' of type '{2}'. Use InlineMethodBody = true and ensure the method has a single-expression or single-return-statement body",
         category: "TUnit.Assertions.SourceGenerator",
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true,
@@ -895,7 +895,7 @@
         // Add null-forgiving operator for reference types if not already present
         // This is safe because we've already checked for null above
         var isNullable = data.TargetType.IsNullable;
         if (isNullable && !string.IsNullOrEmpty(inlinedBody) && !inlinedBody.StartsWith("value!"))
         {
             // Replace null-conditional operators with null-forgiving + regular operators
             // value?.Member becomes value!.Member (safe because we already null-checked)
@@ -1073,7 +1073,7 @@
         }
 
         // Include parameter types to distinguish overloads
-        var paramTypes = string.Join("_", data.AdditionalParameters.Select(p => p.SimpleTypeName));
+        var paramTypes = string.Join('_', data.AdditionalParameters.Select(p => p.SimpleTypeName));
         return $"{targetTypeName}_{methodName}_{paramTypes}_Assertion";
     }
 

TUnit.Assertions/Extensions/AssertionExtensions.cs

@@ -683,7 +683,7 @@ private static string GetMemberPath<TObject, TMember>(Expression<Func<TObject, T
             body = memberExpr.Expression;
         }
 
-        return parts.Count > 0 ? string.Join(".", parts) : "Unknown";
+        return parts.Count > 0 ? string.Join('.', parts) : "Unknown";
     }
 
     /// <summary>

TUnit.Core.SourceGenerator/Extensions/TypeExtensions.cs

@@ -266,6 +266,6 @@ public static string GetNestedClassName(this INamedTypeSymbol typeSymbol)
         typeHierarchy.Reverse();
 
         // Join with '+' separator (matching .NET Type.FullName convention for nested types)
-        return string.Join("+", typeHierarchy);
+        return string.Join('+', typeHierarchy);
     }
 }

TUnit.Core.SourceGenerator/Generators/HookMetadataGenerator.cs

@@ -407,7 +407,7 @@ private static string GetSafeFileName(HookModel hook)
         // Only add if there are parameters (matches main branch behavior - no _0_ suffix for empty params)
         if (hook.Parameters.Length > 0)
         {
-            var paramTypes = string.Join("_", hook.Parameters.Select(p => SanitizeForFileName(GetSimpleTypeName(p.TypeName))));
+            var paramTypes = string.Join('_', hook.Parameters.Select(p => SanitizeForFileName(GetSimpleTypeName(p.TypeName))));
             baseName += $"__{paramTypes}";
         }
 

TUnit.Core.SourceGenerator/Generators/PropertyInjectionSourceGenerator.cs

@@ -1258,7 +1258,7 @@ private static string GetAssemblyQualifiedTypeName(ITypeSymbol typeSymbol)
             }
         }
 
-        return constraintParts.Count > 0 ? string.Join(" ", constraintParts) : null;
+        return constraintParts.Count > 0 ? string.Join(' ', constraintParts) : null;
     }
 
     #endregion

TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs

@@ -3604,7 +3604,7 @@ private static string BuildTypeKey(IEnumerable<ITypeSymbol> types)
         {
             formattedTypes[i] = typesList[i].ToDisplayString(DisplayFormats.FullyQualifiedGenericWithoutGlobalPrefix);
         }
-        return string.Join(",", formattedTypes);
+        return string.Join(',', formattedTypes);
     }
 
     /// <summary>

TUnit.Core/Extensions/ReflectionExtensions.cs

@@ -13,7 +13,7 @@ internal static string GetFormattedName(this Type type)
             return type.Name;
         }
 
-        var genericArguments = string.Join(",", type.GetGenericArguments().Select(GetFormattedName));
+        var genericArguments = string.Join(',', type.GetGenericArguments().Select(GetFormattedName));
 
         var backtickIndex = type.Name.IndexOf("`", StringComparison.Ordinal);
         if (backtickIndex == -1)

TUnit.Core/Extensions/TestContextExtensions.cs

@@ -72,7 +72,7 @@ public static string GetClassTypeName(this TestContext context)
         }
 
         hierarchy.Reverse();
-        return string.Join("+", hierarchy);
+        return string.Join('+', hierarchy);
     }
 
     /// <summary>

TUnit.Core/GenericTestMetadata.cs

@@ -40,7 +40,7 @@ public override Func<ExecutableTestCreationContext, TestMetadata, AbstractExecut
                         {
                             typeNames[i] = inferredTypes[i].FullName ?? inferredTypes[i].Name;
                         }
-                        typeKey = string.Join(",", typeNames);
+                        typeKey = string.Join(',', typeNames);
 
                         // Find the matching concrete instantiation
                         if (genericMetadata.ConcreteInstantiations.TryGetValue(typeKey, out var concreteMetadata))

TUnit.Engine/Discovery/ReflectionHookDiscoveryService.cs

@@ -42,7 +42,7 @@ private static string GetMethodKey(MethodInfo method)
             {
                 paramTypes[i] = parameters[i].ParameterType.FullName ?? "unknown";
             }
-            return $"{m.DeclaringType?.FullName}.{m.Name}({string.Join(",", paramTypes)})";
+            return $"{m.DeclaringType?.FullName}.{m.Name}({string.Join(',', paramTypes)})";
         });
     }
 

TUnit.Engine/Services/FilterParser.cs

@@ -26,7 +26,7 @@ internal class FilterParser
         return filter switch
         {
             NopFilter => null,
-            TestNodeUidListFilter testNodeUidListFilter => string.Join(",",
+            TestNodeUidListFilter testNodeUidListFilter => string.Join(',',
                 testNodeUidListFilter.TestNodeUids.Select(x => x.Value)),
             TreeNodeFilter treeNodeFilter => treeNodeFilter.Filter,
             _ => throw new ArgumentOutOfRangeException(nameof(filter))

TUnit.Engine/Services/TestFilterService.cs

@@ -271,6 +271,6 @@ internal static string GetNestedClassName(ClassMetadata classMetadata)
         }
 
         hierarchy.Reverse();
-        return string.Join("+", hierarchy);
+        return string.Join('+', hierarchy);
     }
 }

TUnit.Mocks.Http/RequestMatcher.cs

@@ -181,8 +181,8 @@ internal string Describe()
         if (_exactPath != null) parts.Add(_exactPath);
         if (_pathPrefix != null) parts.Add($"{_pathPrefix}*");
         if (_pathPattern != null) parts.Add($"~/{_pathPattern}/");
-        if (_requiredHeaders.Count > 0) parts.Add($"headers:{string.Join(",", _requiredHeaders.Keys)}");
+        if (_requiredHeaders.Count > 0) parts.Add($"headers:{string.Join(',', _requiredHeaders.Keys)}");
         if (_bodyContains != null) parts.Add($"body contains \"{_bodyContains}\"");
-        return parts.Count > 0 ? string.Join(" ", parts) : "*";
+        return parts.Count > 0 ? string.Join(' ', parts) : "*";
     }
 }

TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs

@@ -992,7 +992,7 @@ private static string FormatConstraintClauses(EquatableArray<MockTypeParameterMo
                 clauses.Add($"where {tp.Name} : {tp.Constraints}");
             }
         }
-        return clauses.Count > 0 ? " " + string.Join(" ", clauses) : "";
+        return clauses.Count > 0 ? " " + string.Join(' ', clauses) : "";
     }
 
     /// <summary>
@@ -1125,7 +1125,7 @@ public static string GetCompositeSafeName(MockTypeModel model)
         var name = model.FullyQualifiedName;
         if (model.AdditionalInterfaceNames.Length > 0)
         {
-            name += "_" + string.Join("_", model.AdditionalInterfaceNames);
+            name += "_" + string.Join('_', model.AdditionalInterfaceNames);
         }
         return GetSafeName(name);
     }

TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs

@@ -84,7 +84,7 @@ public static (EquatableArray<MockMemberModel> Methods, EquatableArray<MockMembe
 
                     case IPropertySymbol indexer when indexer.IsIndexer:
                     {
-                        var paramTypes = string.Join(",", indexer.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
+                        var paramTypes = string.Join(',', indexer.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
                         var key = $"I:[{paramTypes}]";
                         if (seenProperties.TryGetValue(key, out var existingIndex))
                         {
@@ -183,7 +183,7 @@ public static (EquatableArray<MockMemberModel> Methods, EquatableArray<MockMembe
 
                         case IPropertySymbol indexer when indexer.IsIndexer:
                         {
-                            var paramTypes = string.Join(",", indexer.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
+                            var paramTypes = string.Join(',', indexer.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
                             var key = $"I:[{paramTypes}]";
                             if (seenProperties.TryGetValue(key, out var existingIndex))
                             {
@@ -550,7 +550,7 @@ private static string ComputeUnwrappedSmartDefault(ITypeSymbol returnType, bool
 
     private static string GetMethodKey(IMethodSymbol method)
     {
-        var paramTypes = string.Join(",", method.Parameters.Select(p =>
+        var paramTypes = string.Join(',', method.Parameters.Select(p =>
             p.Type.GetFullyQualifiedName() + (p.RefKind != RefKind.None ? "&" : "")));
         var typeParams = method.TypeParameters.Length > 0 ? $"`{method.TypeParameters.Length}" : "";
         return $"M:{method.Name}{typeParams}({paramTypes})";

TUnit.Mocks.SourceGenerator/Extensions/TypeSymbolExtensions.cs

@@ -118,13 +118,13 @@ private static string GetMemberKey(ISymbol member)
     {
         if (member is IMethodSymbol method)
         {
-            var paramTypes = string.Join(",", method.Parameters.Select(p => p.Type.GetFullyQualifiedName() + (p.RefKind != RefKind.None ? "&" : "")));
+            var paramTypes = string.Join(',', method.Parameters.Select(p => p.Type.GetFullyQualifiedName() + (p.RefKind != RefKind.None ? "&" : "")));
             var typeParams = method.TypeParameters.Length > 0 ? $"`{method.TypeParameters.Length}" : "";
             return $"M:{method.Name}{typeParams}({paramTypes})";
         }
         if (member is IPropertySymbol prop)
         {
-            var paramTypes = string.Join(",", prop.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
+            var paramTypes = string.Join(',', prop.Parameters.Select(p => p.Type.GetFullyQualifiedName()));
             return prop.Parameters.Length > 0 ? $"P:{prop.Name}[{paramTypes}]" : $"P:{prop.Name}";
         }
         if (member is IEventSymbol evt)

TUnit.Mocks/Mock.cs

@@ -286,7 +286,7 @@ public static bool TryCreateAutoMock(Type type, MockBehavior behavior, out IMock
 
     private static string GetMultiKey(params Type[] types)
     {
-        return string.Join("|", types.Select(t => t.FullName ?? t.ToString()));
+        return string.Join('|', types.Select(t => t.FullName ?? t.ToString()));
     }
 
     /// <summary>