using System.Text.RegularExpressions;

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// Specify the invalid filename you want to fix

string fileName = "example@file!name.txt";

​

// Define the regular expression pattern for characters that need to be replaced

string pattern = @"[^\w\-. ]";

​

// Replace invalid characters with valid ones

string fixedFileName = Regex.Replace(fileName, pattern, "_");

​

// Output the original and fixed filenames

Console.WriteLine("Original filename: {0}", fileName);

Console.WriteLine("Fixed filename: {0}", fixedFileName);