Predicate Delegate in C#:

A predicate is also a delegate like Func and Action delegates. It represents a method that contains a set of criteria and checks whether the passed parameter meets those criteria or not. A predicate delegate methods must take one input parameter and it then returns a boolean value - true or false.

The Predicate delegate is defined in the System namespace as shown below:

Predicate signature: public delegate bool Predicate<in T>(T obj);

Same as other delegate types, Predicate can also be used with any method, anonymous method or lambda expression.

Example: Predicate delegate

static bool IsUpperCase(string str)
{
    return str.Equals(str.ToUpper());
}

static void Main(string[] args)
{
    Predicate<string> isUpper = IsUpperCase;

    bool result = isUpper("hello world!!");

    Console.WriteLine(result);
}

Output:
false

An anonymous method can also be assigned to a Predicate delegate type as shown below.

Example: Predicate delegate with anonymous method

static void Main(string[] args)
{
    Predicate<string> isUpper = delegate(string s) { return s.Equals(s.ToUpper());};
    bool result = isUpper("hello world!!");
}

A lambda expression can also be assigned to a Predicate delegate type as shown below.

Example: Predicate delegate with lambda expression

static void Main(string[] args)
{
    Predicate<string> isUpper = s => s.Equals(s.ToUpper());
    bool result = isUpper("hello world!!");
}

Points to Remember :

  1. Predicate delegate takes one input parameter and boolean return type.
  2. Predicate delegate must contains some criateria to check whether supplied parameter meets those criateria or not.
  3. Anonymous method and Lambda expression can be assigned to the predicate delegate.

Learn about an extension method next