Extending IQueryable to return objects where a property contains a string

I see a lot of code similar to the following

var customrs = MyDataContext.Customers.Where(...);
if (!String.IsNullOrEmpty(input)) {
  customers = customers.Where(c => c.Email.Contains(input));
}

I would like to put this in an extension method that checks the input is valid before invoking Where on IQueryable so that it can be called like

customers = MyDataContext.Customers.Where(...)
  .ContainsText(c => c.Email, input);

My extension method looks like this

public static IQueryable<T> ContainsText<T>(this IQueryable<T> source, Expression<Func<T, string>> selector, string text) {
  if (String.IsNullOrEmpty(text) {
    return source;
  }
  else {
    //do something here
  }
}

How can I call Contains() on the expression parsed? Or is there another way to return IQueryable of results where the expression parsed contains the text parsed?

Update: It's for Linq to Sql

tvanfosson had the right idea with building the expression which led to this answer to this question. So for completeness here is a full working solution

The expression builder

public static class ExpressionBuilder {

  public static Expression<Func<T, bool>> ContainsText<T>(string propertyName, string text) {
    var paramExp = Expression.Parameter(typeof(T), "type");
    var propExp = Expression.Property(paramExp, propertyName);
    var methodInfo = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    var valueExp = Expression.Constant(text, typeof(string));
    var methCall = Expression.Call(propExp, methodInfo, valueExp);
    return Expression.Lambda<Func<T, bool>>(methCall, paramExp);
  }

}

The extension method

public static class IQueryableExtensions {

  public static IQueryable<T> ContainsText<T>(this IQueryable<T> source, Expression<Func<T, string>> selector, string text) {
    if (source == null) {
      throw new ArgumentNullException();
    }
    if (text.IsNullOrEmpty()) {
      return source;
    }
    string propName = ((MemberExpression)selector.Body).Member.Name;
    return source.Where(ExpressionBuilder.ContainsText<T>(propName, text));
  }

}

Invoked like

var customers = MyDataContext.Customers.Where(/* some code */)
  .ContainsText(c => c.Email, input);

I think I would make the extension on String, not on IQueryable.

public static bool ContainsIfNotEmpty( this string source, string text )
{
    if (string.IsNullOrEmpty(text))
    {
       return true; // select all elements
    }
    if (string.IsNullOrEmpty(source))
    {
       return false; // select no elements
    }
    return source.Contains(text); // select only matching
}

Then use it as:

customers = MyDataContext.Customers
                         .Where( c => c.Email.ContainsIfNotEmpty( input ) );

Note that this requires LINQ to objects. If you need to use it with LINQ to SQL, then I'd suggest building the expression using a builder method. Note the following is untested as I don't have access to VS right now. You might want to look at Andrew Peters' blog entry for a similar example and/or the documentation on the Expression class.

public static class ExpressionBuilders
{
    public static Expression<Func<T,bool>> ContainsBuilder<T>( string column, string text )
    {
          ParameterExpression parameter = new Expression.Parameter( typeof(T), "t" );

          if (string.IsNullOrEmpty(text))
          {
              return (Expression<Func<T,bool>>)QueryExpression.Lambda( Expression.Constant( true ), parameter );
          }

          MethodInfo contains = typeof(T).GetMethod("Contains");
          Expression textExpression = Expression.Constant(text);
          Expression containsExpression = Expression.Call(parameter,contains,textExpression);

          return (Expression(Func<T,bool>))QueryExpression.Lambda( containsExpression, parameter );
    }
}

Used as:

var predicate = ExpressionBuilders.ContainsBuilder<Customer>( "Email", input );

customers = MyDataContext.Customers.Where( predicate );

The following would be possible using IEnumerable<T>:

public static IEnumerable<T> ContainsText<T>(
    this IEnumerable<T> source, Func<T, string> selector, string text)
{
  return source.Where(
      x => (!string.IsNullOrEmpty(selector(x)) &&
           (selector(x).Contains(text))));
}

In order to stay in the IQueryable<T> your only options would either be Dynamic Linq as tvanfosson has suggested or User Defined Functions.

But to be honest, you might aswell just stay with Where.

I think you can do this instead:

customers = MyDataContext.Customers.Where(c =>  (string.IsNullOrEmpty(input) || c.Email.Contains(input)));