How to Identify the User in Sharepoint Through C# Code

string strUser = txtName.Text.Trim();

SPPrincipalInfo principalInfo = SPUtility.ResolveWindowsPrincipal(mySite.WebApplication, strUser, SPPrincipalType.All, false);

if (principalInfo != null)

{

    SPUser user = myWeb.EnsureUser(strUser);

    ...

    ...

    ...

}

NameSpace:

using Microsoft.SharePoint.Utilities;