Get DescriptionAttribute via reflection

Question

I have this:

public class Demo
{
  [Description("Hello World!")]
  public int SomeProp { get; set; }
}

I want the description. So I wrote this:

public string? GetDescription<TType, TProperty>(Expression<Func<TType, TProperty>> expression)
  where TType : class
{
  var memberName  = ((MemberExpression)expression.Body).Member.Name;
  var description = typeof(TType).GetMember(memberName).First()
                    .GetCustomAttribute<DescriptionAttribute>()
                    ?.Description;
  return description;
}

Which can be used like this:

var description = GetDescription<Demo, string>(x => x.SomeProp);

But what I want is this:

var description = GetDescription<Demo>(x => x.SomeProp);   // no TProperty

I hoped type inference would infer TProperty and simplify the callsite, but I can't compile without it ("Using the generic method requires 2 type arguments").

How can I do this?

Answer 1

One way is to just take a Func<TType, object>. When the property type is not object, the root expression node will be a Convert expression, so you just need to check for that.

public static string? GetDescription<TType>(Expression<Func<TType, object?>> expression)
  where TType : class
{
    var body = expression.Body;
    string memberName;
    if (body is MemberExpression exp1) {
        memberName = exp1.Member.Name;
    } else if (body is UnaryExpression unary && 
               unary.NodeType == ExpressionType.Convert &&
               unary.Operand is MemberExpression exp2) {
        memberName = exp2.Member.Name;
    } else {
        throw new NotSupportedException();
    }
    var description = typeof(TType).GetMember(memberName).First()
                    .GetCustomAttribute<DescriptionAttribute>()
                    ?.Description;
    return description;
}

Or, you can change the use-site and explicitly write the parameter type of the lambda, instead of explicitly writing the type arguments of GetDescription.

GetDescription((Demo x) => x.SomeProp)

---

Side note:

You already got the MemberInfo from the expression, so why not just get the attribute straight from there?

public static string? GetDescription<TType>(Expression<Func<TType, object>> expression)
  where TType : class
{
    var body = expression.Body;
    MemberInfo member;
    if (body is MemberExpression exp1) {
        member = exp1.Member;
    } else if (body is UnaryExpression unary && 
               unary.NodeType == ExpressionType.Convert &&
               unary.Operand is MemberExpression exp2) {
        member = exp2.Member;
    } else {
        throw new NotSupportedException();
    }
    var description = member
                    .GetCustomAttribute<DescriptionAttribute>()
                    ?.Description;
    return description;
}