fixed some bugs in use explicit type context action.

ICSharpCode.NRefactory/CSharp/Refactoring/ContextAction/UseExplicitType.cs

@@ -26,6 +26,8 @@
 using System;
 using System.Linq;
 using ICSharpCode.NRefactory.PatternMatching;
+using ICSharpCode.NRefactory.TypeSystem;
+
 
 namespace ICSharpCode.NRefactory.CSharp.Refactoring
 {
@@ -33,7 +35,11 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 	{
 		public bool IsValid (RefactoringContext context)
 		{
-			return GetVariableDeclarationStatement (context) != null;
+			var varDecl = GetVariableDeclarationStatement (context);
+			if (varDecl == null)
+				return false;
+			var type = context.Resolve (varDecl.Variables.First ().Initializer).Type;
+			return !type.Equals (SharedTypes.Null) && !type.Equals (SharedTypes.UnknownType);
 		}
 		
 		public void Run (RefactoringContext context)
@@ -41,7 +47,8 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 			var varDecl = GetVariableDeclarationStatement (context);
 			
 			using (var script = context.StartScript ()) {
-				script.Replace (varDecl.Type, context.CreateShortType (context.Resolve (varDecl.Variables.First ().Initializer).Type.ConvertToAstType ()));
+				var type = context.Resolve (varDecl.Variables.First ().Initializer).Type;
+				script.Replace (varDecl.Type, context.CreateShortType (type));
 			}
 		}