Question

While doing some basic validation on the ASP.Net (4.0) Request.Files (upload) collection I decided to try it with LINQ.

The collection is IEnumerable<T> and so doesn’t offer ForEach. Foolishly I decided to build an extension method that would do the job. Sorry to say not so much success…

Running the extension method (below) raises next error:

Unable to cast object of type ‘System.String’ to type ‘System.Web.HttpPostedFile’

There is clearly something I am not getting, but I can’t see what it is, so at the risk of looking like an idiot (wont be the first time) here is the code in 3 chunks, along with a promise of gratitude for any help.

First, the extension method with an Action parameter:

//Extend ForEach to IEnumerated Files
public static IEnumerable<HttpPostedFileWrapper> ForEach<T>(this IEnumerable<HttpPostedFileWrapper> source, Action<HttpPostedFileWrapper> action)
{
   //breaks on first 'item' init
   foreach (HttpPostedFileWrapper item in source)
        action(item);
    return source;
}

The error occurs when the internal foreach loop hits the ‘item’ in ‘source’.

Here is the calling code (variables MaxFileTries and attachPath are properly set previously) :

var files = Request.Files.Cast<HttpPostedFile>()
    .Select(file => new HttpPostedFileWrapper(file))
    .Where(file => file.ContentLength > 0
        && file.ContentLength <= MaxFileSize
        && file.FileName.Length > 0)
    .ForEach<HttpPostedFileWrapper>(f => f.SaveUpload(attachPath, MaxFileTries));

And lastly, the Action target, Saving the upload file – we don’t appear to ever even get to here, but just in case, here it is:

public static HttpPostedFileWrapper SaveUpload(this HttpPostedFileWrapper f, string attachPath, int MaxFileTries)
{
    // we can only upload the same file MaxTries times in one session
    int tries = 0;
    string saveName = f.FileName.Substring(f.FileName.LastIndexOf("\\") + 1);   //strip any local
    string path = attachPath + saveName;
    while (File.Exists(path) && tries <= MaxFileTries)
    {
        tries++;
        path = attachPath + " (" + tries.ToString() + ")" + saveName;
    }
    if (tries <= MaxFileTries)
    {
        if (!Directory.Exists(attachPath)) Directory.CreateDirectory(attachPath);
        f.SaveAs(path);
    }
    return f;
}

I confess that some of the above is a cobbling together of “bits found”, so I am likely getting what I deserve, but if anyone has a good understanding of (or has at least been through) this, maybe I can learn something.

Thanks for any.

Answer 1

Why don’t you just call ToList().ForEach() on the original IEnumerable<T>.