.NET 2.0+

The Generic Predicate Delegate

The generic Predicate delegate represents a method or function that accepts an object of any type and returns a Boolean result. It is often used to define a set of criteria for filtering collections such as arrays and generic lists.

What is a Predicate?

A predicate is a logical expression that yields a result of either true or false. Predicates are often used in conditional processing, particularly with "if" statements. For example, in the following C# code the predicate is "value > 10". If the value variable is larger than ten the expression returns true. If it is ten or less, the predicate evaluates as false.

if (value > 10) Console.WriteLine("Large");

The Predicate<T> Delegate

The Predicate<T> delegate is a pre-defined delegate that was introduced in the .NET framework 2.0. It is a generic delegate that is used to reference a method that accepts a parameter of any type and returns a Boolean result. The delegate is used by some of the classes provided by the .NET framework. For example, you can search the contents of arrays and generic lists for items that meet given criteria. The criteria are provided to the appropriate methods using a Predicate<T> parameter.

To demonstrate, add the following code to a console application and execute it in the debugger to see the results. This example creates an integer array containing nine prime numbers. It uses the FindAll method to generate another array containing the prime numbers between zero and nine. The condition is defined in a method that is passed to the second, Predicate<T> argument.

static void Main()
{
    int[] primes = new int[] { 2, 3, 5, 7, 11, 13, 17, 19, 21 };
    int[] smallPrimes = Array.FindAll(primes, IsSingleDigit);   // { 2, 3, 5, 7 }
}

static bool IsSingleDigit(int value)
{
    return 0 <= value && value <= 9;
}

We can simplify the code using an anonymous method for the predicate parameter, as long as it has a single argument and returns a Boolean. The code below is functionally identical to the previous sample.

int[] primes = new int[] { 2, 3, 5, 7, 11, 13, 17, 19, 21 };
int[] smallPrimes = Array.FindAll(primes,
    delegate(int value) { return 0 <= value && value <= 9; });  // { 2, 3, 5, 7 }

NB: When using the .NET framework version 3.5 or later, the functionality provided by the Predicate delegate can be replicated using Language Integrated-Query (LINQ).