C# – LINQ Contains Case Insensitive

You could convert both values to lowercase before doing the comparison. Here's an example for Contains:

case WhereOperation.Contains:
    var toLower = Expression.Call(memberAccessToString,
                                  typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
    condition = Expression.Call(toLower,
                                typeof(string).GetMethod("Contains"),
                                Expression.Constant(value.ToString().ToLower()));
    lambda = Expression.Lambda(condition, parameter);
    break;

Note, however, that this won't pass the Turkey test.

Try this

filteredCategories = categoriesList.Where(c=>
 c.CategoryName.IndexOf("for", StringComparison.OrdinalIgnoreCase) >= 0)

Contains method works as shown below

public bool Contains(string value)
{
   return this.IndexOf(value, StringComparison.Ordinal) >= 0;
}