转换表达式<func<t1, bool> &gt; 到表达式<func<t2, bool> &gt; </func<t2,></func<t1,>

Question

I am using repository pattern. T1 is the Dto of T2 (ex: UserDto and User)

In the service layer, I pass Expression<Func<UserDto, bool>> as a parameter to a method, which in itself calls a method in the data-access layer in which it passes Expression<Func<User, bool>> .

Now, you might ask me why don't I just use Expression<Func<User, bool>> all the way but I can't do that. Is it possible to do the conversion of the Expression? Note that User and UserDTO have the same props.

Answer 1

Yes, as long as you have a method that converts T2 into T1. Here is a simple toy example where T2 is string and T1 is int :

        // That's the conversion function from T2 to T1
        private static int stringToInt(string s) => s.Length;

        [TestMethod]
        public void Test()
        {
            // We want to convert this into string->bool
            Expression<Func<int, bool>> isEven = i => i % 2 == 0;

            MethodInfo stringToIntMethod = ((Func<string, int>) (stringToInt)).Method;

            ParameterExpression x = Expression.Parameter(typeof(string));

            // That's the converted expression
            Expression<Func<string, bool>> converted =
                Expression.Lambda<Func<string, bool>>(
                    Expression.Invoke(isEven,
                        Expression.Call(null, stringToIntMethod, x)), x);

            // Check that it works as expected:
            converted.Compile().Invoke("aa").Should().BeTrue();
            converted.Compile().Invoke("aaa").Should().BeFalse();
        }

If you intend to use the expression with a LINQ provider (eg Entity Framework or Cosmos DB), you must make sure that the entire expression including the conversion call is supported. This is usually not the case if it is a method that you defined.

If that should be problem, can you impose a common interface or base class on both T1 and T2? In that case you could pass an expression of type Expression<Func<BaseClass, bool>> on both layers without conversion. Failing that (eg if T1 and T2 are completely different) the only way to achieve what you want is to analyze the structure of the expression and build a custom expression converter (eg using ExpressionVisitor ).