To check the file signature (also known as the file magic number) and match it with the file extension in C#, you can use the File.ReadAllBytes method to read the first few bytes of the file and compare them with known file signature patterns. Here’s an example code snippet:

using System;
using System.IO;

public class FileSignatureChecker
{
    public static bool IsFileExtensionMatch(string filePath, string expectedExtension)
    {
        // Read the first few bytes of the file
        byte[] fileBytes = new byte[4];
        using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
        {
            fs.Read(fileBytes, 0, fileBytes.Length);
        }

        // Check the file signature against the expected file signature
        if (expectedExtension == ".txt" && IsTextFile(fileBytes))
        {
            return true;
        }
        else if (expectedExtension == ".jpg" && IsJpegFile(fileBytes))
        {
            return true;
        }
        // Add more checks for other file types as needed

        return false; // File signature does not match the expected file extension
    }

    private static bool IsTextFile(byte[] fileBytes)
    {
        // Check for the ASCII text file signature (0x54, 0x45, 0x58, 0x54)
        return fileBytes[0] == 0x54 && fileBytes[1] == 0x45 && fileBytes[2] == 0x58 && fileBytes[3] == 0x54;
    }

    private static bool IsJpegFile(byte[] fileBytes)
    {
        // Check for the JPEG file signature (0xFF, 0xD8, 0xFF)
        return fileBytes[0] == 0xFF && fileBytes[1] == 0xD8 && fileBytes[2] == 0xFF;
    }
}

In this example, the IsFileExtensionMatch method takes the file path and the expected file extension as input. It reads the first four bytes of the file using File.ReadAllBytes and then checks the file signature against the expected file signature for the given file extension.

You can customize the file signature checks in the IsTextFile and IsJpegFile methods based on the file types you want to support. For other file types, you would need to identify their respective file signatures and add the corresponding check methods.

Note that file signatures are not foolproof and can be spoofed. Therefore, it’s always recommended to perform additional validation and security checks when working with file uploads or processing user-supplied files.