Question

Does anyone know why the code below throws System.ArgumentException?

using (var tfc = new TempFileCollection())
{                
    var fn = tfc.AddExtension("tmp");
    Console.WriteLine(fn);
}

Here is exact exception:

System.ArgumentException: The file name 'C:\Users\pczapla\AppData\Local\Temp\iqulrqva.tmp' was already in the collection.
Parameter name: fileName.

Answer 1

A little Reflector action reveals the following interesting snippet in TempFileCollection:

new FileIOPermission(FileIOPermissionAccess.AllAccess, basePath).Demand();
path = this.basePath + ".tmp";
using (new FileStream(path, FileMode.CreateNew, FileAccess.Write))
{
}
flag = true;
...
this.files.Add(path, this.keepFiles);

This is in TempFileCollection.EnsureTempNameCreated, which is called by TempFileCollection.BasePath, which is called by TempFileCollection.AddExtension. I guess the placeholder uses “.tmp” so you cannot.