C#对象初始化时，如何为非空transaction添加Claim

方案一：先构建Claim集合（推荐，逻辑清晰易维护） #

先初始化包含必选Claim的列表，再通过条件判断添加可选的Transaction Claim，最后将集合传入ClaimsIdentity：

var claims = new List<Claim>
{
    new Claim(Constants.JWT_CLAIMS_USER_ID, user.Id.ToString())
};

if (transaction != null)
{
    claims.Add(new Claim(Constants.JWT_CLAIMS_TRANSACTION_GUID, transaction.Guid.ToString()));
}

var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(claims),
    IssuedAt = DateTime.UtcNow,
    Issuer = Constants.JWT_ISSUER,
    Expires = DateTime.UtcNow.AddSeconds(_appSettings.JwtTokenExpirationTime),
    SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};

这种方式可读性强，后续需要添加更多条件Claim时，直接在if块或新增条件分支即可，扩展成本低。

方案二：用LINQ拼接数组（紧凑写法） #

在对象初始化器内，利用Concat方法拼接可选Claim数组，无需额外声明集合变量：

var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(
        new[] { new Claim(Constants.JWT_CLAIMS_USER_ID, user.Id.ToString()) }
        .Concat(transaction != null 
            ? new[] { new Claim(Constants.JWT_CLAIMS_TRANSACTION_GUID, transaction.Guid.ToString()) }
            : Array.Empty<Claim>())
    ),
    IssuedAt = DateTime.UtcNow,
    Issuer = Constants.JWT_ISSUER,
    Expires = DateTime.UtcNow.AddSeconds(_appSettings.JwtTokenExpirationTime),
    SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};

方案三：利用可空元素过滤（C# 8.0+） #

在数组中添加可能为null的Claim，再通过Where过滤掉空元素：

var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(new[] {
        new Claim(Constants.JWT_CLAIMS_USER_ID, user.Id.ToString()),
        transaction != null ? new Claim(Constants.JWT_CLAIMS_TRANSACTION_GUID, transaction.Guid.ToString()) : null
    }.Where(c => c != null)!),
    IssuedAt = DateTime.UtcNow,
    Issuer = Constants.JWT_ISSUER,
    Expires = DateTime.UtcNow.AddSeconds(_appSettings.JwtTokenExpirationTime),
    SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};

注：!运算符用于告诉编译器过滤后不会有null元素，也可以用OfType<Claim>()替代Where(c => c != null)。

内容的提问来源于stack exchange，提问作者BaptX