"The given path's format is not supported."

Question

Rather than using str_uploadpath + fileName, try using System.IO.Path.Combine instead:

Path.Combine(str_uploadpath, fileName);

which returns a string.

I see that the originator found out that the error occurred when trying to save the filename with an entire path. Actually it's enough to have a ":" in the file name to get this error. If there might be ":" in your file name (for instance if you have a date stamp in your file name) make sure you replace these with something else. I.e:

string fullFileName = fileName.Split('.')[0] + "(" + DateTime.Now.ToString().Replace(':', '-') + ")." + fileName.Split('.')[1];

For me the problem was an invisible to human eye "‪" Left-To-Right Embedding character.
It stuck at the beginning of the string (just before the 'D'), after I copy-pasted the path, from the windows file properties security tab.

var yourJson = System.IO.File.ReadAllText(@"D:\test\json.txt"); // Works
var yourJson = System.IO.File.ReadAllText(@"‪D:\test\json.txt"); // Error

So those, identical at first glance, two lines are actually different.

If you are trying to save a file to the file system. Path.Combine is not bullet proof as it won't help you if the file name contains invalid characters. Here is an extension method that strips out invalid characters from file names:

public static string ToSafeFileName(this string s)
{
        return s
            .Replace("\\", "")
            .Replace("/", "")
            .Replace("\"", "")
            .Replace("*", "")
            .Replace(":", "")
            .Replace("?", "")
            .Replace("<", "")
            .Replace(">", "")
            .Replace("|", "");
    }

And the usage can be:

Path.Combine(str_uploadpath, fileName.ToSafeFileName());

Among other things that can cause this error:

You cannot have certain characters in the full PathFile string.

For example, these characters will crash the StreamWriter function:

"/"  
":"

there may be other special characters that crash it too. I found this happens when you try, for example, to put a DateTime stamp into a filename:

AppPath = Path.GetDirectoryName(giFileNames(0))  
' AppPath is a valid path from system. (This was easy in VB6, just AppPath = App.Path & "\")
' AppPath must have "\" char at the end...

DateTime = DateAndTime.Now.ToString ' fails StreamWriter... has ":" characters
FileOut = "Data_Summary_" & DateTime & ".dat"
NewFileOutS = Path.Combine(AppPath, FileOut)
Using sw As StreamWriter = New StreamWriter(NewFileOutS  , True) ' true to append
        sw.WriteLine(NewFileOutS)
        sw.Dispose()
    End Using

One way to prevent this trouble is to replace problem characters in NewFileOutS with benign ones:

' clean the File output file string NewFileOutS so StreamWriter will work
 NewFileOutS = NewFileOutS.Replace("/","-") ' replace / with -
 NewFileOutS = NewFileOutS.Replace(":","-") ' replace : with - 

' after cleaning the FileNamePath string NewFileOutS, StreamWriter will not throw an (Unhandled) exception.

Hope this saves someone some headaches...!

If you get this error in PowerShell, it's most likely because you're using Resolve-Path to resolve a remote path, e.g.

 Resolve-Path \\server\share\path

In this case, Resolve-Path returns an object that, when converted to a string, doesn't return a valid path. It returns PowerShell's internal path:

> [string](Resolve-Path \\server\share\path)
Microsoft.PowerShell.Core\FileSystem::\\server\share\path

The solution is to use the ProviderPath property on the object returned by Resolve-Path:

> Resolve-Path \\server\share\path | Select-Object -ExpandProperty PRoviderPath
\\server\share\path
> (Resolve-Path \\server\share\path).ProviderPath
\\server\share\path

Try changing:

Server.MapPath("/UploadBucket/Raw/")

to

Server.MapPath(@"\UploadBucket\Raw\")