Returning Expression<> using various class properties

Question

I have something like this:

public Expression<Func<Message, bool>> FilterData()
{
    switch (this.operatorEnum)
    {
        case FilterParameterOperatorEnum.EqualTo:
            return message => !string.IsNullOrEmpty(message.Body) &&
                              message.Body
                                     .Equals(this.value, StringComparison.InvariantCultureIgnoreCase);

        case FilterParameterOperatorEnum.NotEqualTo:

            return message => !string.IsNullOrEmpty(message.Body) &&
                              !message.Body
                                     .Equals(this.value, StringComparison.InvariantCultureIgnoreCase);

        case FilterParameterOperatorEnum.Contains:

            return message =>
                    !string.IsNullOrEmpty(message.Body) &&
                    message.Body.IndexOf(this.value,
                                   StringComparison.InvariantCultureIgnoreCase) >= 0;

        case FilterParameterOperatorEnum.DoesNotContain:
            return message =>
                    !string.IsNullOrEmpty(message.Body) &&
                    message.Body.IndexOf(this.value,
                                   StringComparison.InvariantCultureIgnoreCase) == -1;


    }
}

As you can see this is done on Message.Body

I now what to do the same thing on other string properties on the Message class and I don't want to duplicate all that code.

Is there a way to do that by passing in the property somehow?

Answer 1

Eclude the expression that retrieves the property value into a separate lambda expression:

public Expression<Func<Message, bool>> FilterData(Func<Message, string> retrievePropValueFunc)

In your filter expressions, you can then call that new lambda expression (just showing one as an example):

return message => !string.IsNullOrEmpty(retrievePropValueFunc(message))
        && retrievePropValueFunc(message)
                .Equals(this.value, StringComparison.InvariantCultureIgnoreCase);

To get to the Body property, pass message => message.Body to the retrievePropValueFunc parameter; as you see, you can modify this to pass different lambda expressions for the retrieval of other properties just as well.