This is my simple test code. I want to create field assignment link between 2 objects, and the field is determined at runtime using reflection by caching delegate of its setter/getter method. But, somehow it doesn't work. The assignment is not working; maybe I made a silly mistake. Where am I am wrong?
public static class AssignmentExpression
{
public static Expression Create(Expression left, Expression right)
{
MethodInfo m = typeof(AssignmentExpression)
.GetMethod("AssignTo", BindingFlags.NonPublic | BindingFlags.Static)
.MakeGenericMethod(left.Type);
return Expression.Call( null,m,left, right);
}
private static void AssignTo<T>(ref T left, T right)
{
left = right;
}
}
public class FieldLink
{
protected Delegate srcGetter;
protected Delegate dstSetter;
public FieldLink(FieldInfo srcObject, FieldInfo dstObject)
{
this.srcGetter = FieldLink.createGetter(srcObject);
this.dstSetter = FieldLink.createSetter(dstObject);
}
public void update<T>(T dst, T src)
{
this.dstSetter.DynamicInvoke(dst, this.srcGetter.DynamicInvoke(src));
}
protected static Delegate createGetter(FieldInfo field)
{
ParameterExpression objParm = Expression.Parameter(field.DeclaringType, "obj");
Type delegateType = typeof(Func<,>).MakeGenericType(field.DeclaringType, field.FieldType);
MemberExpression fieldExpr = Expression.Field(objParm, field.Name);
LambdaExpression lambda = Expression.Lambda(delegateType, fieldExpr, objParm);
return lambda.Compile();
}
protected static Delegate createSetter(FieldInfo field)
{
ParameterExpression objParm = Expression.Parameter(field.DeclaringType, "obj");
ParameterExpression valueParm = Expression.Parameter(field.FieldType, "value");
Type delegateType = typeof(Action<,>).MakeGenericType(field.DeclaringType, field.FieldType);
MemberExpression memberExpr = Expression.Field(objParm, field.Name);
Expression assignExpr = AssignmentExpression.Create(memberExpr, valueParm);
LambdaExpression lambda = Expression.Lambda(delegateType, assignExpr, objParm, valueParm);
return lambda.Compile();
}
}
public class Test
{
public int fieldInt = 0;
}
public class TestClass
{
public Test a = new Test();
public Test b = new Test();
public void Start()
{
a.fieldInt = 5;
Debug.Log("before a = " + a.fieldInt + " b = " + b.fieldInt);
FieldLink testLink = new FieldLink(this.a.GetType().GetField("fieldInt"),
this.b.GetType().GetField("fieldInt"));
testLink.update(this.b, this.a);
Debug.Log("after a = " + a.fieldInt + " b = " + b.fieldInt);
//here a.fieldInt should be equal to b.fieldInt, but somehow its unchanged!
}
}
Your code appears to work, but maybe not exactly how you're expecting. When you call update
you pass in b.fieldInt
as the left
argument and a.fieldInt
as the right
argument; the update
method then assigns the value of a.fieldInt
(5) to the b.fieldInt
field, resulting in both objects having a fieldInt
value of 5. If you reverse the arguments, both fields end up as zero. Isn't that what you expect?
As an aside - and maybe you have other reasons for using your own AssignmentExpression
class - but you can actually replace:
Expression assignExpr = AssignmentExpression.Create(memberExpr, valueParm);
...with:
Expression assignExpr = Expression.Assign(memberExpr, valueParm);
...and you get the same result.