Fix constants.

Mono.Cecil/Mono.Cecil/FieldDefinition.cs

@@ -124,7 +124,7 @@ namespace Mono.Cecil {
 
 		public bool HasConstant {
 			get {
-				ResolveConstant ();
+				this.ResolveConstant (ref constant, Module);
 
 				return constant != Mixin.NoValue;
 			}
@@ -136,14 +136,6 @@ namespace Mono.Cecil {
 			set { constant = value; }
 		}
 
-		void ResolveConstant ()
-		{
-			if (constant != Mixin.NotResolved)
-				return;
-
-			this.ResolveConstant (ref constant, Module);
-		}
-
 		public bool HasCustomAttributes {
 			get {
 				if (custom_attributes != null)

Mono.Cecil/Mono.Cecil/IConstantProvider.cs

@@ -44,10 +44,14 @@ namespace Mono.Cecil {
 			ref object constant,
 			ModuleDefinition module)
 		{
+			lock (module.SyncRoot) {
+				if (constant != Mixin.NotResolved)
+					return;
 				if (module.HasImage ())
-				module.Read (ref constant, self, (provider, reader) => reader.ReadConstant (provider));
+					constant = module.Read (self, (provider, reader) => reader.ReadConstant (provider));
 				else
 					constant = Mixin.NoValue;
 			}
 		}
 	}
+}

Mono.Cecil/Mono.Cecil/ParameterDefinition.cs

@@ -51,7 +51,7 @@ namespace Mono.Cecil {
 
 		public bool HasConstant {
 			get {
-				ResolveConstant ();
+				this.ResolveConstant (ref constant, parameter_type.Module);
 
 				return constant != Mixin.NoValue;
 			}
@@ -63,14 +63,6 @@ namespace Mono.Cecil {
 			set { constant = value; }
 		}
 
-		void ResolveConstant ()
-		{
-			if (constant != Mixin.NotResolved)
-				return;
-
-			this.ResolveConstant (ref constant, parameter_type.Module);
-		}
-
 		public bool HasCustomAttributes {
 			get {
 				if (custom_attributes != null)

Mono.Cecil/Mono.Cecil/PropertyDefinition.cs

@@ -168,7 +168,7 @@ namespace Mono.Cecil {
 
 		public bool HasConstant {
 			get {
-				ResolveConstant ();
+				this.ResolveConstant (ref constant, Module);
 
 				return constant != Mixin.NoValue;
 			}
@@ -180,14 +180,6 @@ namespace Mono.Cecil {
 			set { constant = value; }
 		}
 
-		void ResolveConstant ()
-		{
-			if (constant != Mixin.NotResolved)
-				return;
-
-			this.ResolveConstant (ref constant, Module);
-		}
-
 		#region PropertyAttributes
 
 		public bool IsSpecialName {