handle case where IL has a string set to an sbyte value.

Mono.Cecil/AssemblyWriter.cs

@@ -1905,7 +1905,10 @@ static ElementType GetConstantType (TypeReference constant_type, object constant
 
 				return ElementType.Class;
 			case ElementType.String:
-				return ElementType.String;
+				if (constant is string)
+					return ElementType.String;
+				else
+					return GetConstantType (constant.GetType ());
 			case ElementType.Object:
 				return GetConstantType (constant.GetType ());
 			case ElementType.Array:
@@ -2201,7 +2204,7 @@ SignatureWriter GetConstantSignature (ElementType type, object value)
 				signature.WriteInt32 (0);
 				break;
 			case ElementType.String:
-				signature.WriteConstantString ((string) value);
+				signature.WriteConstantString (value);
 				break;
 			default:
 				signature.WriteConstantPrimitive (value);
@@ -2904,10 +2907,13 @@ bool TryWriteElementType (TypeReference type)
 			return true;
 		}
 
-		public void WriteConstantString (string value)
+		public void WriteConstantString (object value)
 		{
 			if (value != null)
-				WriteBytes (Encoding.Unicode.GetBytes (value));
+				if (value is string v)
+					WriteBytes (Encoding.Unicode.GetBytes (v));
+				else
+					WritePrimitiveValue (value);
 			else
 				WriteByte (0xff);
 		}

Test/Mono.Cecil.Tests/FieldTests.cs

@@ -341,6 +341,12 @@ public void ConstantCoalescing ()
 				Assert.IsTrue (field.HasConstant);
 				Assert.IsInstanceOf (typeof (char), field.Constant);
 				Assert.AreEqual ('s', field.Constant);
+
+				field = fields.GetField ("string_sbyte");
+				Assert.AreEqual ("System.String", field.FieldType.FullName);
+				Assert.IsTrue (field.HasConstant);
+				Assert.IsInstanceOf (typeof (sbyte), field.Constant);
+				Assert.AreEqual ((sbyte)0x20, field.Constant);
 			});
 		}
 

Test/Resources/il/types.il

@@ -43,4 +43,5 @@
 	.field private static literal char char_int16 = int16(0x0001)
 	.field private static literal int16 int16_char = char(0x0073)
 	.field private static literal int32 int32_nullref = nullref
+	.field private static literal string string_sbyte = int8(0x20)
 }