Fixed cyclic constants.

ICSharpCode.NRefactory.CSharp/Resolver/CSharpConversions.cs

@@ -651,7 +651,10 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 				long val = (long)rr.ConstantValue;
 				return val >= 0 && toTypeCode == TypeCode.UInt64;
 			} else if (fromTypeCode == TypeCode.Int32) {
-				int val = (int)rr.ConstantValue;
+				object cv = rr.ConstantValue;
+				if (cv == null)
+					return false;
+				int val = (int)cv;
 				switch (toTypeCode) {
 					case TypeCode.SByte:
 						return val >= SByte.MinValue && val <= SByte.MaxValue;

ICSharpCode.NRefactory.Tests/CSharp/Parser/TypeSystemConvertVisitorTests.cs

@@ -97,6 +97,7 @@ namespace ICSharpCode.NRefactory.CSharp.Parser
 			Assert.NotNull (testType);
 			var field = testType.Fields.First ();
 			Assert.IsTrue (field.IsConst);
+			Assert.IsNull (field.ConstantValue);
 		}
 	}
 	

ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultResolvedField.cs

@@ -19,6 +19,7 @@
 using System;
 using System.Collections.Generic;
 using ICSharpCode.NRefactory.Semantics;
+using ICSharpCode.NRefactory.Utils;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 {
@@ -49,6 +50,9 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 		
 		public object ConstantValue {
 			get {
+				using (var busyLock = BusyManager.Enter(this)) {
+					if (!busyLock.Success)
+						return null;
 					ResolveResult rr = this.constantValue;
 					if (rr == null) {
 						IConstantValue unresolvedCV = ((IUnresolvedField)unresolved).ConstantValue;
@@ -62,4 +66,5 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 				}
 			}
 		}
+	}
 }