BUGFIX IDISP003 don't warn when assigned with null.

Fix #197

Author: JohanLarsson

IDisposableAnalyzers.Test/IDISP003DisposeBeforeReassigningTests/Valid.cs

@@ -70,6 +70,30 @@ public void M()
             RoslynAssert.Valid(Analyzer, code);
         }
 
+        [Test]
+        public static void LocalAssignDisposeAssignNullAssign()
+        {
+            var code = @"
+namespace N
+{
+    using System;
+    using System.IO;
+
+    public class C
+    {
+        public void M()
+        {
+            var stream = File.OpenRead(string.Empty);
+            stream.Dispose();
+            stream = null;
+            stream = File.OpenRead(string.Empty);
+        }
+    }
+}";
+
+            RoslynAssert.Valid(Analyzer, code);
+        }
+
         [Test]
         public static void LocalAssignedInSwitch()
         {
@@ -378,6 +402,32 @@ public C()
             RoslynAssert.Valid(Analyzer, code);
         }
 
+        [Test]
+        public static void FieldAssignDisposeAssignNullAssignInCtor()
+        {
+            var code = @"
+namespace N
+{
+    using System;
+    using System.IO;
+
+    public class C
+    {
+        private readonly Stream stream;
+
+        public C()
+        {
+            this.stream = File.OpenRead(string.Empty);
+            this.stream.Dispose();
+            this.stream = null;
+            this.stream = File.OpenRead(string.Empty);
+        }
+    }
+}";
+
+            RoslynAssert.Valid(Analyzer, code);
+        }
+
         [Test]
         public static void FieldSwapCached()
         {

IDisposableAnalyzers/Helpers/Disposable.IsDisposed.cs

@@ -52,12 +52,14 @@ static bool TryGetScope(SyntaxNode node, out BlockSyntax result)
                     result = lambdaBody;
                     return true;
                 }
-                else if (node.FirstAncestor<AccessorDeclarationSyntax>() is { Body: BlockSyntax accessorBody })
+
+                if (node.FirstAncestor<AccessorDeclarationSyntax>() is { Body: BlockSyntax accessorBody })
                 {
                     result = accessorBody;
                     return true;
                 }
-                else if (node.FirstAncestor<BaseMethodDeclarationSyntax>() is { Body: BlockSyntax methodBody })
+
+                if (node.FirstAncestor<BaseMethodDeclarationSyntax>() is { Body: BlockSyntax methodBody })
                 {
                     result = methodBody;
                     return true;
@@ -69,16 +71,17 @@ static bool TryGetScope(SyntaxNode node, out BlockSyntax result)
 
             bool IsReassignedAfter(SyntaxNode scope, InvocationExpressionSyntax disposeCall)
             {
-                using (var walker = MutationWalker.Borrow(scope, SearchScope.Member, semanticModel, cancellationToken))
+                using var walker = AssignmentWalker.Borrow(scope);
+                foreach (var assignment in walker.Assignments)
                 {
-                    foreach (var mutation in walker.All())
+                    if (assignment.TryFirstAncestor(out StatementSyntax? statement) &&
+                        disposeCall.IsExecutedBefore(statement) == ExecutedBefore.Yes &&
+                        statement.IsExecutedBefore(expression) == ExecutedBefore.Yes &&
+                        semanticModel.TryGetSymbol(assignment.Left, cancellationToken, out var assigned) &&
+                        Equals(assigned, symbol) &&
+                        IsCreation(assignment.Right, semanticModel, cancellationToken).IsEither(Result.Yes, Result.AssumeYes))
                     {
-                        if (mutation.TryFirstAncestor(out StatementSyntax? statement) &&
-                            disposeCall.IsExecutedBefore(statement) == ExecutedBefore.Yes &&
-                            statement.IsExecutedBefore(expression) == ExecutedBefore.Yes)
-                        {
-                            return true;
-                        }
+                        return true;
                     }
                 }