Why we use underscore before variable name in C# ?

A lot of C# programmers like to precede field names with an underscore and a lower case letter to distinguish them from an associated property which begins with an upper case letter. For example:

private string _name;

public string Name
{
    get { return _name;}
    set { _name = value; }
}

This is just a convention and you don't have to follow it if you're allergic to underscores.

One thing you shouldn't do is to precede your identifiers with a double underscore as this is reserved for use by the system. For example __arglist is an undocumented C# keyword which is used for calling C functions which take a variable number of  arguments.