Visual Studio has an out-of-the box plugin that generates GUIDs for you. However its a WebTest plugin, so it will only generate them once per webtest. You can’t include that plugin into a loop.

Fortunately we can write a simple WebTestRequest plugin that does what we want:

using System;
using System.ComponentModel;
using Microsoft.VisualStudio.TestTools.WebTesting;
using Microsoft.VisualStudio.TestTools.WebTesting.Rules;

namespace LoadTestPlugins
{
    [DisplayNameAttribute("Generate GUID")]
    [DescriptionAttribute("Generates a GUID during a request")]
    public class GenerateGUID : WebTestRequestPlugin
    {

        #region Methods
        private void GenerateIt(WebTestContext Context)
        {
            Context[this.ContextParameterName] = Guid.NewGuid().ToString();
        }

        public override void PostRequest(object sender, PostRequestEventArgs e)
        {
            if(this.BeforeRequest == false)
            {
                this.GenerateIt(e.WebTest.Context);
            }
        }

        public override void PreRequest(object sender, PreRequestEventArgs e)
        {
            if(this.BeforeRequest == true)
            {
                this.GenerateIt(e.WebTest.Context);
            }
        }
        #endregion

        #region Properties
        [Description("Name of context parameter to store the number in"),
        DisplayName("Context parameter"), IsContextParameterName(true)]
        public string ContextParameterName {get; set;}

        [Description("Whether to generate the number before or after the request has executed"),
        DisplayName("Apply before request"),
        DefaultValue(true)]
        public bool BeforeRequest { get; set; }
        #endregion
    }
}