使用委托進行字段設置/通知

Question

這是我的簡單測試代碼。 我想在2個對象之間創建字段分配鏈接，並在運行時使用反射通過緩存其setter / getter方法的委托來確定該字段。 但是，以某種方式它不起作用。 分配工作失敗； 也許我犯了一個愚蠢的錯誤。 我哪里錯了？

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!
    }
}

Answer 1

您的代碼似乎可以正常工作，但可能與您的期望不完全一樣。 調用update您將b.fieldInt作為left參數傳遞，並將a.fieldInt作為right參數a.fieldInt ； 然后， update方法將a.fieldInt （5）的值分配給b.fieldInt字段，導致兩個對象的fieldInt值均為5。如果反轉參數，則兩個字段最終都為零。 那不是您所期望的嗎？

順便說一句-也許您還有其他使用自己的AssignmentExpression類的原因-但您實際上可以替換：

Expression assignExpr = AssignmentExpression.Create(memberExpr, valueParm);

...具有：

Expression assignExpr = Expression.Assign(memberExpr, valueParm);

...您得到相同的結果。