Delegates in C#

Delegates can be used to hold the reference of method. A delegate is a type that safety encapsulates a method and it is similar to function pointer in C and C++. Delegates can be used to define call back methods. Delegate class hold only those methods which have same signature. It allow methods to be passed as parameters. A delegate can be seen as a placeholder for method.

Delegates can be chained multiple methods together and can be called on a single event.

Suppose if you have multiple methods with same signature which have same return type & number of parameters and want to call all the methods with single object then we can go for delegates.

Creating and using delegates involve the four steps:

1. Declaring a delegates

2. Defining a delegate method

3. Creating delegate object

4. Invoking delegate object

    class Program

    {

        // Step-1. Declaring a delegates

        // Access modifier - delegate keyword - return type - delegate-name

        public delegate void Calculate(int a, int b);

        // Step-2. Defining a delegate method

        // Signature of method exactly matches the delegate signature.

        public static void Add(int x, int y)

        {

            Console.WriteLine("Sum = {0}", x + y);

        }

        static void Main(string[] args)

        {

            // Step-3. Creating delegate object

            Calculate calculate = new Calculate(Add);

            // Step-4. Invoking delegate object

            calculate(5, 5);

            Console.ReadKey();

        }

       

    }

You can also assign any method to degates like :

// Step-3. Creating delegate object

  Calculate calculate = Add;

Types of Delegates

1. Single cast delegate

Delegate that represent only a single method is known as Single Cast Delegate.

    class Program

    {

        //  Declaring a delegates

       

        public delegate void Calculate(int a, int b);

        // Defining a delegate method

       

        public static void Add(int x, int y)

        {

            Console.WriteLine("Sum = {0}", x + y);

        }

        static void Main(string[] args)

        {

            // Creating delegate object

            Calculate calculate = new Calculate(Add);

            // Invoking delegate object

            calculate(5, 5);

            Console.ReadKey();

        }

       

    }

2. Multi Cast Delegate

Delegate that represent more than one method is known as Multi Cast Delegate.

Use += operator to add reference of more method to delegate

Use -= operator to remove reference of a method

    class Program

    {

        // Declaring a delegates       

        public delegate void Calculate(int a, int b);

        // Defining a delegate method       

        public static void Add(int x, int y)

        {

            Console.WriteLine("Sum = {0}", x + y);

        }

        // Defining second delegate method

        public static void Multiply(int x, int y)

        {

            Console.WriteLine("Multiply = {0}", x + y);

        }

  

        static void Main(string[] args)

        {

            // Creating delegate object

            Calculate calculate = new Calculate(Add);

            // Add second method to delegate

            calculate += Multiply;

            // Invoking delegate object, it will call Add and Multiply both method

            calculate(5, 5);

            Console.ReadKey();

        }
   }

You can also create separate delegate object for each method and combine all objects to a single to perform Multicast delegate.

   // Alternate way of creating Multicast delegate object

   Calculate delegate1 = Add;

   Calculate delegate2 = Multiply;

   Calculate calculate = delegate1 + delegate2;

Remove reference of a method from Multicast delegate object.

        static void Main(string[] args)

        {

            //Creating delegate object

            Calculate calculate = new Calculate(Add);

            // add second method to delegate

            calculate += Multiply;

            // Remove reference of Add method to delegate

            calculate -= Add;

            // Invoking delegate object, it will call only Multiply method

            calculate(5, 5);

            Console.ReadKey();

        }

        // Output

        // Multiply = 10

Update Image in GridView in Asp.Net.

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default3.aspx.cs" Inherits="Default3" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head runat="server">

    <title></title>

</head>

<body>

    <form id="form1" runat="server">

    <div>

        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" AllowPaging="True"

            OnRowCancelingEdit="GridView1_RowCancelingEdit" DataKeyNames="ID" CellPadding="4"

            OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating" ForeColor="#333333">

            <Columns>

                <asp:TemplateField HeaderStyle-Width="150px">

                    <ItemTemplate>

                        <asp:LinkButton ID="LkB1" runat="server" CommandName="Edit">Edit</asp:LinkButton>

                    </ItemTemplate>

                    <EditItemTemplate>

                        <asp:LinkButton ID="LkB2" runat="server" CommandName="Update">Update</asp:LinkButton>

                        <asp:LinkButton ID="LkB3" runat="server" CommandName="Cancel">Cancel</asp:LinkButton>

                    </EditItemTemplate>

                </asp:TemplateField>

                <asp:TemplateField HeaderText="Name" HeaderStyle-Width="200px">

                    <ItemTemplate>

                        <asp:Label ID="Label1" runat="server" Text='<%# Eval("Name") %>'></asp:Label>

                    </ItemTemplate>

                    <EditItemTemplate>

                        <asp:TextBox ID="txt_Name" runat="server" Text='<%# Eval("Name") %>'></asp:TextBox>

                    </EditItemTemplate>

                </asp:TemplateField>

                <asp:TemplateField HeaderText="Image" HeaderStyle-Width="200px">

                    <ItemTemplate>

                        <asp:Image ID="Image1" runat="server" ImageUrl='<%# Eval("Image") %>' Height="80px"

                            Width="100px" />

                    </ItemTemplate>

                    <EditItemTemplate>

                        <asp:Image ID="img_user" runat="server" ImageUrl='<%# Eval("Image") %>' Height="80px"

                            Width="100px" /><br />

                        <asp:FileUpload ID="FileUpload1" runat="server" />

                    </EditItemTemplate>

                </asp:TemplateField>

            </Columns>

            <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />

            <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />

        </asp:GridView>

    </div>

    </form>

</body>

</html>

 =================================================================

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Data.SqlClient;

using System.Data;

public partial class Default3 : System.Web.UI.Page

{

    // sql connection

    SqlConnection con = new SqlConnection(@"Data Source=KCLINK-PC;Initial Catalog=Naresh;Integrated Security=True");

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            BindGridView();

        }

    }

    //method for binding GridView

    protected void BindGridView()

    {

        DataTable dt = new DataTable();

        SqlDataAdapter da = new SqlDataAdapter("Select ID, Name,Branch,City,Image from tbl_student"

                                                                                                                 , con);

        con.Open();

        da.Fill(dt);

        con.Close();

        if (dt.Rows.Count > 0)

        {

            GridView1.DataSource = dt;

            GridView1.DataBind();

        }

    }

    // edit event

    protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)

    {

        GridView1.EditIndex = e.NewEditIndex;

        BindGridView();

    }

    // update event

    protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)

    {

        //find student id of edit row

        string id = GridView1.DataKeys[e.RowIndex].Value.ToString();

        // find values for update

        TextBox name = (TextBox)GridView1.Rows[e.RowIndex].FindControl("txt_Name");

        FileUpload FileUpload1 = (FileUpload)GridView1.Rows[e.RowIndex].FindControl("FileUpload1");

        string path = "~/Images/UserImage/";

        if (FileUpload1.HasFile)

        {

            path += FileUpload1.FileName;

            //save image in folder

            FileUpload1.SaveAs(MapPath(path));

        }

        else

        {

            // use previous user image if new image is not changed

            Image img = (Image)GridView1.Rows[e.RowIndex].FindControl("img_user");

            path = img.ImageUrl;

        }

        SqlCommand cmd = new SqlCommand("update tbl_student set Name='" + name.Text + "',image='"+path+"'  where ID=" + id + "", con);

        con.Open();

        cmd.ExecuteNonQuery();

        con.Close();

        GridView1.EditIndex = -1;

        BindGridView();

    }

    // cancel edit event

    protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)

    {

        GridView1.EditIndex = -1;

        BindGridView();

    }

}