Use of Extension method in Dot Net.

Extension methods are used for the Readability purpose.It's a static method.
In C# we have to create STATIC CLASS but in VB.Net, we have to create a MODULE like below snippets:-

Note:- 
1). In C#, we have to give this keyword in the 1st parameter list
2). In VB.Net,we have to define <System.Runtime.CompilerServices.Extension()> _ attribute in function.

//C#
public static class Text_Validation
{
}

//VB.Net
Public Module Text_Validation
End Module

We can understand this by an example:-

//In C#:-

public static class Text_Validation
{
  public static string Add_Text(this  string str)
  {
     if(string.IsNullOrEmpty(str))
     {
        str = "Add Text";
     } 
     return str;
  }
}

//In VB.Net:-

Public Module Text_Validation

<System.Runtime.CompilerServices.Extension()> _ 
    Public Function Add_Text(ByVal str As String) As String
        If String.IsNullOrEmpty(str) Then
            str = "Add Text"
        End If
        Return str
    End Function
End Module

//Our aspx page will look like this:-
<asp:TextBox ID="txt_contents" runat="server"></asp:TextBox>

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
        txt_contents.Text = txt_contents.Text.Add_Text()
End Sub

//If the TextBox is empty then it will add "Add Text" in TextBox.