Since you're already in Visual Studio, why not just simply write the code?

foreach (var f in new DirectoryInfo(@"...").GetFiles("*.cs", SearchOption.AllDirectories)) {
  string s = File.ReadAllText(f.FullName);
  File.WriteAllText (f.FullName, s, Encoding.UTF8);
}

Only three lines of code! I'm sure you can write this in less than a minute :-)

I would convert the files programmatically (outside VS), e.g. using a Python script:

import glob, codecs

for f in glob.glob("*.py"):
    data = open("f", "rb").read()
    if data.startswith(codecs.BOM_UTF8):
        # Already UTF-8
        continue
    # else assume ANSI code page
    data = data.decode("mbcs")
    data = codecs.BOM_UTF8 + data.encode("utf-8")
    open("f", "wb").write(data)

This assumes all files not in "UTF-8 with signature" are in the ANSI code page - this is the same what VS 2008 apparently also assumes. If you know that some files have yet different encodings, you would have to specify what these encodings are.

Thanks for your solutions, this code has worked for me :

Dim s As String = ""
Dim direc As DirectoryInfo = New DirectoryInfo("Your Directory path")

For Each fi As FileInfo In direc.GetFiles("*.vb", SearchOption.AllDirectories)
    s = File.ReadAllText(fi.FullName, System.Text.Encoding.Default)
    File.WriteAllText(fi.FullName, s, System.Text.Encoding.Unicode)
Next

Convert from UTF-8-BOM to UTF-8

Building on rasx's answer, here is a PowerShell function that assumes your current files are already encoded in UTF-8 (but maybe with BOM) and converts them to UTF-8 without BOM, therefore preserving existing Unicode characters.

Function Write-Utf8([string] $path, [string] $filter='*')
{
    [IO.SearchOption] $option = [IO.SearchOption]::AllDirectories;
    [String[]] $files = [IO.Directory]::GetFiles((Get-Item $path).FullName, $filter, $option);
    foreach($file in $files)
    {
        "Writing $file...";
        [String]$s = [IO.File]::ReadAllText($file, [Text.Encoding]::UTF8);
        [Text.Encoding]$e = New-Object -TypeName Text.UTF8Encoding -ArgumentList ($false);
        [IO.File]::WriteAllText($file, $s, $e);
    }
}

the item is removed from the menu in Visual Studio 2017 You can still access the functionality through File-> Save As -> then clicking the down arrow on the Save button and clicking "Save With Encoding...".

You can also add it back to the File menu through Tools->Customize->Commands if you want to.

I'm only offering this suggestion in case there's no way to automatically do this in Visual Studio (I'm not even sure this would work):

1. Create a class in your project named 足の不自由なハッキング (or some other unicode text that will force Visual Studio to encode as UTF-8).
2. Add "using MyProject.足の不自由なハッキング;" to the top of each file. You should be able to do it on everything by doing a global replace of "using System.Text;" with "using System.Text;using MyProject.足の不自由なハッキング;".
3. Save everything. You may get a long string of "Do you want to save X.cs using UTF-8?" messages or something.