further check invocation symbol (#82)

* further check invocation symbol
- check if symbol is method
- check if multiple candidate symbols are found

* fix NullReference

Author: LukasGelke

System.IO.Abstractions.Analyzers.Tests/Analyzers/PathAnalyzerTests.cs

@@ -12,6 +12,7 @@ public class PathAnalyzerTests : CSharpDiagnosticAnalyzerTest<PathAnalyzer>
 		[Theory]
 		[InlineData("Valid.txt")]
 		[InlineData("UsingStaticFalsePositive.txt")]
+		[InlineData("UsingStaticLinqFalsePositive.txt")]
 		public void Analyzer_is_not_triggered(string filename)
 		{
 			var source = ReadFile(filename);

System.IO.Abstractions.Analyzers.Tests/TestData/Analyzer/PathAnalyzer/UsingStaticLinqFalsePositive.txt

@@ -0,0 +1,29 @@
+namespace SomeNameSpace.Mocking
+{
+	using System.IO.Abstractions;
+	public static class FileSystemProxy
+	{
+		internal static IPath Path { get; }
+	}
+}
+
+namespace SomeNameSpace
+{
+	using System.Collections.Generic;
+	using System.Linq;
+	using static SomeNameSpace.Mocking.FileSystemProxy;
+	public class UsingStaticFalsePositive
+	{
+		private IList<string> files;
+		public UsingStaticFalsePositive()
+		{
+		}
+
+		public void SomeMethod()
+		{
+			var x = files
+				.First(item => Path.GetFileName(item)
+					.Equals("Pattern.txt", StringComparison.OrdinalIgnoreCase));
+		}
+	}
+}

System.IO.Abstractions.Analyzers/Analyzers/BaseFileSystemNodeAnalyzer.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -69,10 +70,30 @@ private bool IsStaticInvocationStartWith(InvocationExpressionSyntax invocation)
 		private static bool IsInvocationFromAbstractions(SyntaxNodeAnalysisContext context, InvocationExpressionSyntax invocation)
 		{
 			return (invocation?.Expression as MemberAccessExpressionSyntax)?.Expression is ExpressionSyntax invokedMember
-					&& context.SemanticModel.GetSymbolInfo(invokedMember).Symbol is ISymbol symbol
-					&& ((symbol as IPropertySymbol)?.Type ?? (symbol as IFieldSymbol)?.Type) is ITypeSymbol type
-					&& !type.ContainingNamespace.IsGlobalNamespace
-					&& type.ContainingNamespace.ToString().StartsWith(Constants.FileSystemNameSpace, StringComparison.Ordinal);
+					&& IsSymbolFromAbstractions(context.SemanticModel.GetSymbolInfo(invokedMember));
+		}
+
+		private static bool IsSymbolFromAbstractions(SymbolInfo symbolInfo)
+		{
+			if (symbolInfo.Symbol is ISymbol symbol)
+			{
+				return IsSymbolFromAbstractions(symbol);
+			} else if (symbolInfo.CandidateSymbols.Length > 0)
+			{
+				return symbolInfo.CandidateSymbols.All(IsSymbolFromAbstractions);
+			}
+			return false;
+		}
+
+		private static bool IsSymbolFromAbstractions(ISymbol symbol)
+		{
+			INamespaceSymbol namespaceSymbol
+				= ((symbol as IPropertySymbol)?.Type ?? (symbol as IFieldSymbol)?.Type)?.ContainingNamespace
+					?? (symbol as IMethodSymbol)?.ContainingNamespace;
+
+			return namespaceSymbol != null
+				&& !namespaceSymbol.IsGlobalNamespace
+				&& namespaceSymbol.ToString().StartsWith(Constants.FileSystemNameSpace, StringComparison.Ordinal);
 		}
 	}
 }