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
    }
}

If a webtest stores temporary data in a context parameter, then you need to make sure that its reset to some initial state when the webtest starts. Otherwise it will fail if that webtest is included multiple times within the same testrun.

Visual Studio’s “Set Context Parameter Value” plug-in won’t help here as it cannot set the value of a parameter to an empty string. If you try this, then during the test run, the request will fail with the error “Request failed: Missing or invalid Context Parameter Name.”

The solution in my case was a very simple custom plug-in. I choose to implement it as a WebTestPlugin instead of a WebTestRequestPlugin to avoid confusing it with Visual Studio’s existing plugin.

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

namespace LoadTestPlugins.WebTestPlugins
{
    [DisplayNameAttribute("Set context parameter value")]
    [DescriptionAttribute("Sets the context parameter to a value")]
    public class ClearContextParameter : WebTestPlugin
    {
        public override void PreWebTest(object sender, PreWebTestEventArgs e)
        {
            e.WebTest.Context[this.ContextParameter] = this.Value;
        }

        #region Properties
        [Description("Context parameter to set"),
        DisplayName("Context parameter"), IsContextParameterName(true)]
        public string ContextParameter { get; set; }

        [Description("Value to place into the context parameter"),
        DisplayName("Value")]
        public string Value { get; set; }
        #endregion
    }
}

An important part of loadtesting with Markov chains is choosing which transition the test should take. This boils down to generating a random number and then seeing in which of the transitions in the probability matrix that number belongs to.

In a previous post I showed how to generate a random number. In this post we’ll see how we can decide which transition to actually take based upon the probabilities from the matrix.

Webtests can include various types of plugins that execute requests based on some boolean condition. These type of plugins are called ConditionalRule plugins. I created a simple plugin that determines if the random number falls within a user specified range. This allows me to implement the following kind of logic in my webtest:

Here is the code for the plugin:

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

namespace LoadTestPlugins
{

    [DisplayNameAttribute("Number in range")]
    [DescriptionAttribute("Determines if the number in the context parameter is in the specified range")]
    public class NumberInRange : ConditionalRule
    {

        #region Methods
        public override void CheckCondition(object sender, ConditionalEventArgs e)
        {
            Int32 Input = Convert.ToInt32(e.WebTest.Context[this.ContextParameterName]);
            e.IsMet = (Input >= this.LowerBound && Input <= this.UpperBound) ? true : false;
            return;
        }

        public override string StringRepresentation()
        {
            return string.Format("{0} (Number in range [{1} - {2}])"
                , this.Description
                , this.LowerBound
                , this.UpperBound);
        }
        
        #endregion

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

        [Description("The lower bound"),
        DisplayName("Lower bound")]
        public Int32 LowerBound { get; set; }


        [Description("The upper (inclusive) bound"),
        DisplayName("Upper bound")]
        public Int32 UpperBound { get; set; }

        [Description("Your description of this rule"),
        DisplayName("Description")]
        public string Description { get; set; }
        
        #endregion
    }
}

An important part of loadtesting with Markov chains is choosing which transition the test should take. This boils down to generating a random number and then seeing in which of the transitions in the probability matrix that number belongs to.

The plugin code below generates a random number between your specified lower- and upper bound and places it into the specified context parameter. Its very similar to the out-of-the box plugin. However Visual Studio’s plugin can only be executed once when a webtest starts. I needed a new random number every time that my loops executes.

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

namespace LoadTestPlugins
{

    [DisplayNameAttribute("Generate random number")]
    [DescriptionAttribute("Generates a random number during a request")]
    public class GenerateRandomNumber : WebTestRequestPlugin
    {

        #region Methods
        private void GenerateIt(WebTestContext Context)
        {
            Context[this.ContextParameterName] = new Random().Next(this.LowerBound, this.UpperBound);
        }

        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("The lower bound"),
        DisplayName("Lower bound"),
        DefaultValue(1)]
        public Int32 LowerBound { get; set; }


        [Description("The upper (inclusive) bound"),
        DisplayName("Upper bound"),
        DefaultValue(100)]
        public Int32 UpperBound { 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
    }
}