Validate data from telegram web app (https://core.telegram.org/bots/webapps#validating-data-received-via-the-web-app) in C#.

ValidateWebData.cs

// Parse string initData from telegram.
var data = HttpUtility.ParseQueryString(initData);

// Put data in a alphabetically sorted dict.
var dataDict = new SortedDictionary<string, string>(
    data.AllKeys.ToDictionary(x => x!, x => data[x]!),
    StringComparer.Ordinal);

// Constant key to genrate secret key.
var constantKey = "WebAppData";

// https://core.telegram.org/bots/webapps#validating-data-received-via-the-web-app:
// Data-check-string is a chain of all received fields,
// sorted alphabetically.
// in the format key=<value>.
// with a line feed character ('\n', 0x0A) used as separator.
// e.g., 'auth_date=<auth_date>\nquery_id=<query_id>\nuser=<user>'
var dataCheckString = string.Join(
    '\n', dataDict.Where(x => x.Key != "hash") // Hash should be removed.
        .Select(x => $"{x.Key}={x.Value}")); // like auth_date=<auth_date> ..

// secrecKey is the HMAC-SHA-256 signature of the bot's token
// with the constant string WebAppData used as a key.
var secretKey = HMACSHA256.HashData(
    Encoding.UTF8.GetBytes(constantKey), // WebAppData
    Encoding.UTF8.GetBytes(_botConfig.BotToken)); // Bot's token

var generatedHash = HMACSHA256.HashData(
    secretKey,
    Encoding.UTF8.GetBytes(dataCheckString)); // data_check_string

// Convert received hash from telegram to a byte array.
var actualHash = Convert.FromHexString(dataDict["hash"]); // .NET 5.0 

// Compare our hash with the one from telegram.
if (actualHash.SequenceEqual(generatedHash))
{
    // Data from telegram.
}

Thanks, This really helped