Usage of Expression<Func<T>>

Question

I am developing data-interfacing that converts between two different data models. However, I must be sure that all required fields exist. Therefore I have written this utility class that I can easily use to verify required fields.

However I am unsure whether this is the best way because of the expression that needs to be compiled and the usage of reflection. Any feedback is welcome!

Usage

 public OutputDataElement DetermineRetailTransactionShopperType(IHeaderEntity headerEntity)
 {
 Ensure.IsNotNull(() => headerEntity);
 Ensure.IsNotNull(() => headerEntity.ShopId, "headerEntity");
 // Some mapping logic removed from the example
 } 

Utility

/// <summary>
/// Helper class able to ensure expectations
/// </summary>
public static class Ensure
{
 public static void IsNotNull<T>(Expression<Func<T>> property) where T : class
 {
 IsNotNullImpl(property, null);
 }
 public static void IsNotNull<T>(Expression<Func<T>> property, string paramName) where T : class
 {
 IsNotNullImpl(property, paramName);
 }
 private static void IsNotNullImpl<T>(Expression<Func<T>> property, string paramName) where T : class
 {
 // Compile the linq expression
 Func<T> compiledFunc = property.Compile();
 // Invoke the linq expression to get the value
 T fieldValue = compiledFunc.Invoke();
 // Check whether we have a value
 if ((fieldValue is string && fieldValue.ToString() == string.Empty) || (fieldValue == null))
 {
 // We have no value. Get the initial expression
 var expression = (MemberExpression)property.Body;
 // log information about the expression that failed
 throw new ArgumentException(string.Format("Missing required field '{0}'", expression.Member.Name), string.IsNullOrEmpty(paramName) ? null : paramName);
 }
 }
}

Answer 1

I don't see any unnecessary reflection in your code.

One think that could be simplified is your check for string.Empty:

if (fieldValue == null || fieldValue as string == string.Empty)

• Alain: Hmm, Just because fieldValue != null, doesn't mean you can cast it as a string without throwing an exception.
• svick: I just simplified the code that was in the question. I don't understand why would it have to throw an exception.
• Alain: He checked fieldValue is string before casting it to a string. If you cast something that's not a string to a string it's not the same as calling Object.ToString() - it will cause an exception.
• svick: That's exactly why I'm using as. It will never throw an exception. If the object is a different type, it will return null.