Add null node for VariableInitializer.

ICSharpCode.NRefactory.CSharp/Ast/AstNode.cs

@@ -671,7 +671,7 @@ namespace ICSharpCode.NRefactory.CSharp
 			public static readonly Role<TypeParameterDeclaration> TypeParameter = new Role<TypeParameterDeclaration> ("TypeParameter");
 			public static readonly Role<AstType> TypeArgument = new Role<AstType> ("TypeArgument", CSharp.AstType.Null);
 			public readonly static Role<Constraint> Constraint = new Role<Constraint> ("Constraint");
-			public static readonly Role<VariableInitializer> Variable = new Role<VariableInitializer> ("Variable");
+			public static readonly Role<VariableInitializer> Variable = new Role<VariableInitializer> ("Variable", VariableInitializer.Null);
 			public static readonly Role<Statement> EmbeddedStatement = new Role<Statement> ("EmbeddedStatement", CSharp.Statement.Null);
 			public static readonly Role<CSharpTokenNode> Keyword = new Role<CSharpTokenNode> ("Keyword", CSharpTokenNode.Null);
 			public static readonly Role<CSharpTokenNode> InKeyword = new Role<CSharpTokenNode> ("InKeyword", CSharpTokenNode.Null);

ICSharpCode.NRefactory.CSharp/Ast/Statements/VariableDeclarationStatement.cs

@@ -63,7 +63,7 @@ namespace ICSharpCode.NRefactory.CSharp
 		
 		public VariableInitializer GetVariable (string name)
 		{
-			return Variables.FirstOrDefault (vi => vi.Name == name);
+			return Variables.FirstOrNullObject (vi => vi.Name == name);
 		}
 		
 		public override S AcceptVisitor<T, S> (IAstVisitor<T, S> visitor, T data = default(T))

ICSharpCode.NRefactory.CSharp/Ast/TypeMembers/VariableInitializer.cs

@@ -28,6 +28,29 @@ namespace ICSharpCode.NRefactory.CSharp
 {
 	public class VariableInitializer : AstNode
 	{
+		#region Null
+		public new static readonly VariableInitializer Null = new NullVariableInitializer ();
+		
+		sealed class NullVariableInitializer : VariableInitializer
+		{
+			public override bool IsNull {
+				get {
+					return true;
+				}
+			}
+			
+			public override S AcceptVisitor<T, S> (IAstVisitor<T, S> visitor, T data = default(T))
+			{
+				return default (S);
+			}
+			
+			protected internal override bool DoMatch(AstNode other, PatternMatching.Match match)
+			{
+				return other == null || other.IsNull;
+			}
+		}
+		#endregion
+
 		#region PatternPlaceholder
 		public static implicit operator VariableInitializer(PatternMatching.Pattern pattern)
 		{