Delegates and Types of Delegates in C#

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Table Of Contents

• Why Delegates?
• Definition
• Types of Delegate
• Single Delegate
• Multicast Delegate
• Generic Delegate
1. Func
2. Action
3. Predicate
• Differences among Generic Delegates
• Implementation of Generic Delegates in Expression Tree

Why Delegates?

Delegates are used in the following cases:

• Delegates can be used to handle(call/invoke) multiple methods on a single event.
• Delegates can be used to define callback(asynchronous) methods.
• Delegates can be used for decoupling and implementing generic behaviors.
• Delegates can be invoked method at runtime.
• Delegates can be used in LINQ for parsing the ExpressionTree.
• Delegates can be used in different Design Pattern.

Definition

A delegate(known as function pointer in C/C++) is a references type that invokes single/multiple method(s) through the delegate instance. It holds a reference of the methods. Delegate types are sealed and immutable type.

Types of Delegates

There are three types of delegates that can be used in C#.

• Single Delegate
• Multicast Delegate
• Generic Delegate

Single Delegate

Single delegate can be used to invoke a single method. In the given source code example, a delegate CalculateSimpleInterest invokes a method getTotalInterest().

    /// Sample example of simple delegate
    /// 
    class Program
    {
       // Declare a delegate
        delegate double CalculateSimpleInterest(double p, double t, double r);
        static CalculateSimpleInterest SI = getTotalInterest;

        static void Main(string[] args)
        {
            double totalInterest;
            //Method I : Invocation of simple delegate by using Invoke keyword
            totalInterest = SI.Invoke(120, 1, 3.25);
            Console.WriteLine("Total Interest of $120 
            in a year at rate of 3.25% APR is {0}",totalInterest);

            //Method II : Invocation of simple delegate by passing method name
            CalculateSimpleInterest D = new CalculateSimpleInterest(getTotalInterest);
            totalInterest = D(120, 1, 3.25);
            Console.WriteLine("Total Interest of $120 
            in a year at rate of 3.25% APR is {0}", totalInterest);
            Console.ReadKey();
        }
//Creating methods which will be assigned to delegate object
        /// <summary>
        /// Gets the total interest.
        /// </summary>
        /// <param name="p" />The Principal.
        /// <param name="t" />The Time.
        /// <param name="r" />The Rate.
        /// <returns>Total Interest
        static double getTotalInterest(double p, double t, double r)
        {
            return (p * t * r) / 100;
        }
    }

Multicast Delegate

Multicast delegate can be used to invoke the multiple methods. The delegate instance can do multicasting (adding new method on existing delegate instance) using the + operator and – operator can be used to remove a method from a delegate instance. All methods will invoke in sequence as they are assigned.

In the given source code example, a delegate instance dObjSI invokes the methods getTotalInterest(), getInterestRatePerYear() and getInterestTimeSpan().

/// <summary>
/// Sample example of multicast delegate
///
class Program
{
   // Declare a delegate
    delegate double CalculateSimpleInterest(double para1, double para2, double para3);
    static CalculateSimpleInterest dObjSI = getTotalInterest;

    static void Main(string[] args)
    {
        double SI;
        //Calculating simple interest
        SI = dObjSI.Invoke(120, 1, 3.25);
        //using multicast delegate by invoking method getInterestRatePerYear()
        dObjSI += new CalculateSimpleInterest(getInterestRatePerYear);
        double Rate=dObjSI.Invoke(SI, 120, 1);
        Console.WriteLine("APR rate is {0}", Rate);
        //using multicast delegate by invoking method getInterestTimeSpan()
        dObjSI += new CalculateSimpleInterest(getInterestTimeSpan);
        double TimeSpan = dObjSI.Invoke(SI, 120, 3.25);
        Console.WriteLine("Time Span is {0}", TimeSpan);


        Console.ReadKey();
    }

    /// <summary>
    /// Gets the total interest.
    /// </summary>
    /// <param name="p" />The Principal.
    /// <param name="t" />The Time.
    /// <param name="r" />The Rate.
    /// <returns>Total Interest
    static double getTotalInterest(double p, double t, double r)
    {
        return (p * t * r) / 100;
    }
    /// <summary>
    /// Gets the interest rate per year.
    /// </summary>
    /// <param name="SI" />The Simple Interest.
    /// <param name="p" />The Principal.
    /// <param name="t" />The Time.
    /// <returns>Interest rate per year
    static double getInterestRatePerYear(double SI, double p, double t)
    {
        return (SI * 100)/(p*t);
    }
    /// <summary>
    /// Gets the interest time span.
    /// </summary>
    /// <param name="SI" />The Simple Interest.
    /// <param name="p" />The Principal.
    /// <param name="r" />The Rate.
    /// <returns>Interest time span
    static double getInterestTimeSpan(double SI, double p, double r)
    {
        return (SI * 100) / (p * r);
    }
}

Generic Delegate

Generic Delegate was introduced in .NET 3.5 that don't require to define the delegate instance in order to invoke the methods.

There are three types of generic delegates:

• Func
• Action
• Predicate

Generic Delegate: Func

The Func delegate defines a method that can be called on arguments and returns a result. In the given code example, delegate Func<interest,double> is defined with Interest type as argument and double as return type.

    /// <summary>
    /// Sample example of generic delegate
    /// 
    class Program
    {
       // Declare a delegate
        delegate double CalculateSimpleInterest(double para1, double para2, double para3);
        static CalculateSimpleInterest dObjSI = getTotalInterest;

        static void Main(string[] args)
        {
           double SI;
            //Declare a generic Func delegate
            Func<interest,double> calcSI = SIObj =>(SIObj.P*SIObj.T*SIObj.R)/100;
            Interest obj = new Interest();
            obj.P = 120; obj.T = 1; obj.R = 3.25;
            // Consuming delegate
            SI = calcSI(obj);
            Console.WriteLine("Total Interest of $120 in a year at rate of 3.25% APR is {0}", SI);
            Console.ReadKey();
        }       
    }
    class Interest
    {
        public double P { get; set; }
        public double T { get; set; }
        public double R { get; set; }
    }
</interest,double>

Generic Delegate: Action

The Action delegate defines a method that can be called on arguments but does not return a result. In the given code example, delegate Action<string> is defined with string as argument.

Action<string> MyAction = y => Console.Write(y);
            MyAction("Hello");
            Console.ReadKey();

Generic Delegate: Predicate

The Predicate delegate defines a method that can be called on arguments and always returns Boolean type result. In the given code example, delegate Predicate<string> checkValidDate is defined with string type as argument and returns bool type.

    /// <summary>
    /// Sample example of generic delegate: Predicate
    /// 
    class Program
    {
        static void Main(string[] args)
        {
            string date="05/12/20143";
            Predicate<string> checkValidDate = d => IsDate(d) ;
            if (checkValidDate(date))
            {
                Console.WriteLine("Valid Date");
            }
            else
            {
                Console.WriteLine("Invalid Date");
            }
            Console.ReadKey();            
        }
         private static bool IsDate(string date)
         {
             DateTime dt;
             return DateTime.TryParse(date,out dt);
         }

    }
</string>

Expression Tree

Expression trees allow you to build code dynamically at runtime instead of statically typing it in the IDE and using a compiler. Expression Trees use generic delegates to create and parse the expressions.

Expression trees are used in the following cases:

• Expression trees can be used to create LINQ to SQL and EF to SQL.
• Expression trees can be used for ASP.NET MVC's HTML extensions.
• Expression trees can be used to determine the selected property or field in MVC.

In the given code example, a expression (3+5)-(4-2) is divided into three expressions as Exp1 for (3+5), Exp2 for (4-2) and Exp3 for adding Exp1 and Exp2. The expression Expression.Lambda<func<int>>(resultexp).compile()() uses Func generic delegate to parse the expressions.

/// <summary>
    /// Sample example of Expression Tree
    /// 
    class Program
    {
        static void Main(string[] args)
        {
            //Express tree (3+5)-(4-2)
            //3+5
            BinaryExpression Exp1 = Expression.MakeBinary(ExpressionType.Add, Expression.Constant(3),
                Expression.Constant(5));
            //4-2
            BinaryExpression Exp2 = Expression.MakeBinary(ExpressionType.Subtract, Expression.Constant(4),
                Expression.Constant(2));
           // (3+5)-(4-2)
            BinaryExpression resultExp = Expression.MakeBinary(ExpressionType.Subtract, Exp1, Exp2);
            //this stmt will create a delegates by parsing the expression three
            int result = Expression.Lambda<func<int>>
            (resultexp).compile()(); console.writeline("result="{0}",">

Difference Between Each Type of Generic Delegate

History

• 13^th May, 2014: Initial version