using (SPWeb site = this.Web) 
{ 
    // Validate the page request to avoid 
    // any malicious posts. 
    if (Request.HttpMethod == "POST") 
    SPUtility.ValidateFormDigest(); 

    // Get a reference to the roles that 
    // are bound to the user and the role 
    // definition against which we need to 
    // verify the user. 
    SPRoleDefinitionBindingCollection usersRoles = 
      site.AllRolesForCurrentUser; 
    SPRoleDefinitionCollection siteRoleCollection = 
      site.RoleDefinitions; 
    SPRoleDefinition roleDefinition = 
      siteRoleCollection["Full Control"]; 
    // Determine whether the user is in the role. If 
    // not, redirect the user to the access-denied page 
    if (usersRoles.Contains(roleDefinition)) 
    { 
        //************************************ 
        // Check whether post back to run 
        // code that initiates the page. 
        if (IsPostBack == true) 
        { 
            // Execute application page logic. 
        } 
    } 
    else 
    { 
        Response.Redirect("/_layouts/accessdenied.aspx"); 
    } 
}

http://msdn.microsoft.com/en-us/library/dd878359(v=office.12).aspx

Check if user has a specific Role Definition

security, sharepoint

C#