How to check if File exists in C#? (Inside folder)

You can simple call File.Exists(path), where path can contain folder or complete directory url, which will return true if file exists or false it doesn't exists.

var FileURL=@"C:\DriveFolder\file.txt";
if(!File.Exists(FileURL))
{
  //create file, as files doesn't exists.
}

Note: Passing an invalid path to .Exists method, it returns false

Also it will return false, if user doesn't have permission to read file in the specified file path url.

You can also check if file exists using FileInfo

DirectoryInfo di = new DirectoryInfo(ProcessingDirectory);
FileInfo[] myFiles = di.GetFiles("*.txt");
if (myFiles.Length == 0)
{
    Console.WriteLine("No File");
}
foreach (var fInfo in myFiles)   
{
    if (!fInfo.Exists)
    {
        throw new FileNotFoundException("The file was not found.", FileName);
    }
}

You can also read, more about file operations in C# here "File I/O in C#"