From https://stackoverflow.com/questions/23158379/how-to-generate-json-postman-collections-from-a-webapi2-project-using-webapi-hel

PostmanCollectionGet

/// <summary>
///     [Postman](http://getpostman.com) collection representation
/// </summary>
public class PostmanCollectionGet
{
    /// <summary>
    ///     Id of collection
    /// </summary>
    [JsonProperty(PropertyName = "id")]
    public Guid Id { get; set; }

    /// <summary>
    ///     Name of collection
    /// </summary>
    [JsonProperty(PropertyName = "name")]
    public string Name { get; set; }

    /// <summary>
    ///     Collection generation time
    /// </summary>
    [JsonProperty(PropertyName = "timestamp")]
    public long Timestamp { get; set; }

    /// <summary>
    ///     Requests associated with the collection
    /// </summary>
    [JsonProperty(PropertyName = "requests")]
    public ICollection<PostmanRequestGet> Requests { get; set; }

    /// <summary>
    ///     **unused always false**
    /// </summary>
    [JsonProperty(PropertyName = "synced")]
    public bool Synced { get; set; }

    /// <summary>
    ///     folders within the collection
    /// </summary>
    [JsonProperty(PropertyName = "folders")]
    public ICollection<PostmanFolderGet> Folders { get; set; }

    /// <summary>
    ///     Description of collection
    /// </summary>
    [JsonProperty(PropertyName = "description")]
    public string Description { get; set; }
}

PostmanController

/// <summary>
///     Based on
///     http://blogs.msdn.com/b/yaohuang1/archive/2012/06/15/using-apiexplorer-to-export-api-information-to-postman-a-chrome-extension-for-testing-web-apis.aspx
/// </summary>
[RoutePrefix("api/postman")]
public class PostmanApiController : ApiController
{
    /// <summary>
    ///     Produce [POSTMAN](http://www.getpostman.com) related responses
    /// </summary>
    public PostmanApiController()
    {
        // exists for documentation purposes
    }

    private readonly Regex _pathVariableRegEx = new Regex("\\{([A-Za-z0-9-_]+)\\}", RegexOptions.ECMAScript | RegexOptions.Compiled);
    private readonly Regex _urlParameterVariableRegEx = new Regex("=\\{([A-Za-z0-9-_]+)\\}", RegexOptions.ECMAScript | RegexOptions.Compiled);

    /// <summary>
    ///     Get a postman collection of all visible Api
    ///     (Get the [POSTMAN](http://www.getpostman.com) chrome extension)
    /// </summary>
    /// <returns>object describing a POSTMAN collection</returns>
    /// <remarks>Get a postman collection of all visible api</remarks>
    [HttpGet]
    [Route(Name = "GetPostmanCollection")]
    [ResponseType(typeof (PostmanCollectionGet))]
    public IHttpActionResult GetPostmanCollection()
    {
        return Ok(this.PostmanCollectionForController());
    }

    private PostmanCollectionGet PostmanCollectionForController()
    {
        var requestUri = Request.RequestUri;
        var baseUri = requestUri.Scheme + "://" + requestUri.Host + ":" + requestUri.Port
                      + HttpContext.Current.Request.ApplicationPath;

        var postManCollection = new PostmanCollectionGet
                                {
                                    Id = Guid.NewGuid(),
                                    Name = "[Name of your API]",
                                    Timestamp = DateTime.Now.Ticks,
                                    Requests = new Collection<PostmanRequestGet>(),
                                    Folders = new Collection<PostmanFolderGet>(),
                                    Synced = false,
                                    Description = "[Description of your API]"
                                };


        var helpPageSampleGenerator = Configuration.GetHelpPageSampleGenerator();

        var apiExplorer = Configuration.Services.GetApiExplorer();

        var apiDescriptionsByController = apiExplorer.ApiDescriptions.GroupBy(
            description =>
            description.ActionDescriptor.ActionBinding.ActionDescriptor.ControllerDescriptor.ControllerType);

        foreach (var apiDescriptionsByControllerGroup in apiDescriptionsByController)
        {
            var controllerName = apiDescriptionsByControllerGroup.Key.Name.Replace("Controller", string.Empty);

            var postManFolder = new PostmanFolderGet
                                {
                                    Id = Guid.NewGuid(),
                                    CollectionId = postManCollection.Id,
                                    Name = controllerName,
                                    Description = string.Format("Api Methods for {0}", controllerName),
                                    CollectionName = "api",
                                    Order = new Collection<Guid>()
                                };

            foreach (var apiDescription in apiDescriptionsByControllerGroup
                .OrderBy(description => description.HttpMethod, new HttpMethodComparator())
                .ThenBy(description => description.RelativePath)
                .ThenBy(description => (description.Documentation ?? "").ToString(CultureInfo.InvariantCulture)))
            {
                TextSample sampleData = null;
                var sampleDictionary = helpPageSampleGenerator.GetSample(apiDescription, SampleDirection.Request);
                MediaTypeHeaderValue mediaTypeHeader;
                if (MediaTypeHeaderValue.TryParse("application/json", out mediaTypeHeader)
                    && sampleDictionary.ContainsKey(mediaTypeHeader))
                {
                    sampleData = sampleDictionary[mediaTypeHeader] as TextSample;
                }

                // scrub curly braces from url parameter values
                var cleanedUrlParameterUrl = this._urlParameterVariableRegEx.Replace(apiDescription.RelativePath, "=$1-value");

                // get pat variables from url
                var pathVariables = this._pathVariableRegEx.Matches(cleanedUrlParameterUrl)
                                        .Cast<Match>()
                                        .Select(m => m.Value)
                                        .Select(s => s.Substring(1, s.Length - 2))
                                        .ToDictionary(s => s, s => string.Format("{0}-value", s));

                // change format of parameters within string to be colon prefixed rather than curly brace wrapped
                var postmanReadyUrl = this._pathVariableRegEx.Replace(cleanedUrlParameterUrl, ":$1");

                // prefix url with base uri
                var url = baseUri.TrimEnd('/') + "/" + postmanReadyUrl;

                var request = new PostmanRequestGet
                              {
                                  CollectionId = postManCollection.Id,
                                  Id = Guid.NewGuid(),
                                  Name = apiDescription.RelativePath,
                                  Description = apiDescription.Documentation,
                                  Url = url,
                                  Method = apiDescription.HttpMethod.Method,
                                  Headers = "Content-Type: application/json",
                                  Data = sampleData == null
                                             ? null
                                             : sampleData.Text,
                                  DataMode = "raw",
                                  Time = postManCollection.Timestamp,
                                  Synced = false,
                                  DescriptionFormat = "markdown",
                                  Version = "beta",
                                  Responses = new Collection<string>(),
                                  PathVariables = pathVariables
                              };

                postManFolder.Order.Add(request.Id); // add to the folder
                postManCollection.Requests.Add(request);
            }

            postManCollection.Folders.Add(postManFolder);
        }

        return postManCollection;
    }
}

/// <summary>
///     Quick comparer for ordering http methods for display
/// </summary>
internal class HttpMethodComparator : IComparer<HttpMethod>
{
    private readonly string[] _order =
    {
        "GET",
        "POST",
        "PUT",
        "DELETE"
    };

    public int Compare(HttpMethod x, HttpMethod y)
    {
        return Array.IndexOf(this._order, x.ToString()).CompareTo(Array.IndexOf(this._order, y.ToString()));
    }
}

PostmanFolder

/// <summary>
///     Object that describes a [Postman](http://getpostman.com) folder
/// </summary>
public class PostmanFolderGet
{
    /// <summary>
    ///     id of the folder
    /// </summary>
    [JsonProperty(PropertyName = "id")]
    public Guid Id { get; set; }

    /// <summary>
    ///     folder name
    /// </summary>
    [JsonProperty(PropertyName = "name")]
    public string Name { get; set; }

    /// <summary>
    ///     folder description
    /// </summary>
    [JsonProperty(PropertyName = "description")]
    public string Description { get; set; }

    /// <summary>
    ///     ordered list of ids of items in folder
    /// </summary>
    [JsonProperty(PropertyName = "order")]
    public ICollection<Guid> Order { get; set; }

    /// <summary>
    ///     Name of the collection
    /// </summary>
    [JsonProperty(PropertyName = "collection_name")]
    public string CollectionName { get; set; }

    /// <summary>
    ///     id of the collection
    /// </summary>
    [JsonProperty(PropertyName = "collection_id")]
    public Guid CollectionId { get; set; }
}

PostmanRequest

/// <summary>
///     [Postman](http://getpostman.com) request object
/// </summary>
public class PostmanRequestGet
{
    /// <summary>
    ///     id of request
    /// </summary>
    [JsonProperty(PropertyName = "id")]
    public Guid Id { get; set; }

    /// <summary>
    ///     headers associated with the request
    /// </summary>
    [JsonProperty(PropertyName = "headers")]
    public string Headers { get; set; }

    /// <summary>
    ///     url of the request
    /// </summary>
    [JsonProperty(PropertyName = "url")]
    public string Url { get; set; }

    /// <summary>
    ///     path variables of the request
    /// </summary>
    [JsonProperty(PropertyName = "pathVariables")]
    public Dictionary<string, string> PathVariables { get; set; }

    /// <summary>
    ///     method of request
    /// </summary>
    [JsonProperty(PropertyName = "method")]
    public string Method { get; set; }

    /// <summary>
    ///     data to be sent with the request
    /// </summary>
    [JsonProperty(PropertyName = "data")]
    public string Data { get; set; }

    /// <summary>
    ///     data mode of reqeust
    /// </summary>
    [JsonProperty(PropertyName = "dataMode")]
    public string DataMode { get; set; }

    /// <summary>
    ///     name of request
    /// </summary>
    [JsonProperty(PropertyName = "name")]
    public string Name { get; set; }

    /// <summary>
    ///     request description
    /// </summary>
    [JsonProperty(PropertyName = "description")]
    public string Description { get; set; }

    /// <summary>
    ///     format of description
    /// </summary>
    [JsonProperty(PropertyName = "descriptionFormat")]
    public string DescriptionFormat { get; set; }

    /// <summary>
    ///     time that this request object was generated
    /// </summary>
    [JsonProperty(PropertyName = "time")]
    public long Time { get; set; }

    /// <summary>
    ///     version of the request object
    /// </summary>
    [JsonProperty(PropertyName = "version")]
    public string Version { get; set; }

    /// <summary>
    ///     request response
    /// </summary>
    [JsonProperty(PropertyName = "responses")]
    public ICollection<string> Responses { get; set; }

    /// <summary>
    ///     the id of the collection that the request object belongs to
    /// </summary>
    [JsonProperty(PropertyName = "collection-id")]
    public Guid CollectionId { get; set; }

    /// <summary>
    ///     Synching
    /// </summary>
    [JsonProperty(PropertyName = "synced")]
    public bool Synced { get; set; }
}