<p>I am trying to fill a form in a php application from a C# client (Outlook addin). I used Fiddler to see the original request from within the php application and the form is transmitted as a multipart/form. Unfortunately .Net does not come with native support for this type of forms (WebClient has only a method for uploading a file).
Does anybody know a library or has some code to achieve this? I want to post different values and additionally (but only sometimes) a file.</p>

<p>Thanks for your help,
Sebastian</p>

This is cut and pasted from some sample code I wrote, hopefully it should give the basics. It only supports File data and form-data at the moment.
```
```
public class PostData
{

    private List<PostDataParam> m_Params;

    public List<PostDataParam> Params
    {
        get { return m_Params; }
        set { m_Params = value; }
    }

    public PostData()
    {
        m_Params = new List<PostDataParam>();

        // Add sample param
        m_Params.Add(new PostDataParam("email", "MyEmail", PostDataParamType.Field));
    }

    /// <summary>
    /// Returns the parameters array formatted for multi-part/form data
    /// </summary>
    /// <returns></returns>
    public string GetPostData()
    {
        // Get boundary, default is --AaB03x
        string boundary = ConfigurationManager.AppSettings["ContentBoundary"].ToString();

        StringBuilder sb = new StringBuilder();
        foreach (PostDataParam p in m_Params)
        {
            sb.AppendLine(boundary);

            if (p.Type == PostDataParamType.File)
            {
                sb.AppendLine(string.Format("Content-Disposition: file; name=\"{0}\"; filename=\"{1}\"", p.Name, p.FileName));
                sb.AppendLine("Content-Type: text/plain");
                sb.AppendLine();
                sb.AppendLine(p.Value);                 
            }
            else
            {
                sb.AppendLine(string.Format("Content-Disposition: form-data; name=\"{0}\"", p.Name));
                sb.AppendLine();
                sb.AppendLine(p.Value);
            }
        }

        sb.AppendLine(boundary);

        return sb.ToString();           
    }
}

public enum PostDataParamType
{
    Field,
    File
}

public class PostDataParam
{

    public PostDataParam(string name, string value, PostDataParamType type)
    {
        Name = name;
        Value = value;
        Type = type;
    }

    public string Name;
    public string FileName;
    public string Value;
    public PostDataParamType Type;
}
```
```
To send the data you then need to:
```
```
HttpWebRequest oRequest = null;
oRequest = (HttpWebRequest)HttpWebRequest.Create(oURL.URL);
oRequest.ContentType = "multipart/form-data";                       
oRequest.Method = "POST";
PostData pData = new PostData();

byte[] buffer = encoding.GetBytes(pData.GetPostData());

// Set content length of our data
oRequest.ContentLength = buffer.Length;

// Dump our buffered postdata to the stream, booyah
oStream = oRequest.GetRequestStream();
oStream.Write(buffer, 0, buffer.Length);
oStream.Close();

// get the response
oResponse = (HttpWebResponse)oRequest.GetResponse();
```
```
Hope thats clear, i've cut and pasted from a few sources to get that tidier.