Fix null-reference exception when writing to console (#48)

Author: ErikSchierboom

src/FluentAssertions.Analyzers.Tests/DiagnosticVerifier.cs

@@ -28,6 +28,7 @@ static DiagnosticVerifier()
             References = new[]
             {
                 typeof(object), // System.Private.CoreLib
+                typeof(Console), // System
                 typeof(Enumerable), // System.Linq
                 typeof(CSharpCompilation), // Microsoft.CodeAnalysis.CSharp
                 typeof(Compilation), // Microsoft.CodeAnalysis

src/FluentAssertions.Analyzers.Tests/Tips/SanityTests.cs

@@ -90,5 +90,21 @@ public static void Main() { }
 
             DiagnosticVerifier.VerifyCSharpDiagnosticUsingAllAnalyzers(source);
         }
+
+        [TestMethod]
+        [Implemented(Reason = "https://github.com/fluentassertions/fluentassertions.analyzers/issues/49")]
+        public void WritingToConsole_ShouldNotThrow()
+        {
+            const string source = @"
+public class TestClass
+{
+    public static void Main()
+    {
+        System.Console.WriteLine();
+    }
+}";
+            
+            DiagnosticVerifier.VerifyCSharpDiagnosticUsingAllAnalyzers(source);
+        }    
     }
 }

src/FluentAssertions.Analyzers/Utilities/FluentAssertionsAnalyzer.cs

@@ -58,6 +58,7 @@ protected virtual Diagnostic AnalyzeExpression(ExpressionSyntax expression, Sema
             var variableNameExtractor = new VariableNameExtractor(semanticModel);
             expression.Accept(variableNameExtractor);
 
+            if (variableNameExtractor.VariableIdentifierName == null) return null;
             var typeInfo = semanticModel.GetTypeInfo(variableNameExtractor.VariableIdentifierName);
             if (typeInfo.ConvertedType == null) return null;
             if (!ShouldAnalyzeVariableType(typeInfo.ConvertedType)) return null;