Expression for Type members results in different Expressions (MemberExpression, UnaryExpression)

Question

Description

I have a expression to point on a property of my type. But it does not work for every property type. \"Does not mean\" means it result in different exp

Answer 1

The reason this happens is that Age is a value type. In order to coerce an expression returning a value type into Func<Person,object> the compiler needs to insert a Convert(expr, typeof(object)), a UnaryExpression.

For strings and other reference types, however, there is no need to box, so a "straight" member expression is returned.

If you would like to get to the MemberExpression inside the UnaryExpression, you can get its operand:

private static MemberExpression GetMemberExpression<T>(
    Expression<Func<T,object>> exp
) {
    var member = exp.Body as MemberExpression;
    var unary = exp.Body as UnaryExpression;
    return member ?? (unary != null ? unary.Operand as MemberExpression : null);
}

Answer 2

Rather than comparing the Member.Name string, I would suggest comparing the PropertyInfo instances directly for equality, in order to avoid false positives when two properties in distinct classes share the same name.

public static bool IsSameProperty<TSourceA, TSourceB, TPropertyA, TPropertyB>(
    Expression<Func<TSourceA, TPropertyA>> expA,
    Expression<Func<TSourceB, TPropertyB>> expB)
{
    MemberExpression memExpA = expA.Body as MemberExpression;
    MemberExpression memExpB = expB.Body as MemberExpression;

    if (memExpA == null || memExpB == null)
        return false;

    PropertyInfo propA = memExpA.Member as PropertyInfo;
    PropertyInfo propB = memExpB.Member as PropertyInfo;

    if (propA == null || propB == null)
        return false;

    return propA.Equals(propB);
}

You can ensure that your lambda expression is compiled as a MemberExpression rather than a UnaryExpression simply by specifying the correct value type (rather than object) as the generic type TResult of your Expression<Func<T, TResult>> expression.

Expression<Func<Person, int>> expression1 = x => x.Age;
Expression<Func<Person, int>> expression2 = x => x.Age;
Expression<Func<Person, string>> expression3 = x => x.Name;

Console.WriteLine(IsSameProperty(expression1, expression2));   // True
Console.WriteLine(IsSameProperty(expression1, expression3));   // False