JWT Decoder for C# / .NET Developers

// Install: dotnet add package System.IdentityModel.Tokens.Jwt
using Microsoft.IdentityModel.Tokens;
using System.IdentityModel.Tokens.Jwt;
using System.Text;

var secret = "your-256-bit-secret-key-here-min-32";
var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(secret));

// Validate token
var handler = new JwtSecurityTokenHandler();
var validationParams = new TokenValidationParameters {
    ValidateIssuerSigningKey = true,
    IssuerSigningKey = key,
    ValidateIssuer = false,
    ValidateAudience = false,
};

var principal = handler.ValidateToken(token, validationParams, out var validated);
var userId = principal.FindFirst("userId")?.Value;