我如何将方法“utilities.DecryptStringFromBase64String”的调用限制为一次

Question

我从 web 服务调用之一获取凭证数据，解密时，格式为“用户名：：：密码”。 我还有其他 class，最后我正在创建数据 class，如下所示，

 var lstStudents = new List<Student>
        {
            new Student
            {
                Name = "studen1",
                Credentials = new List<Credential> {new Credential {Key = "N1", Cred = "pNn/B3yUB+x2yiC310efoAjb8EkNhH1oD3NYF0v5SNxUKPtOtpxL21saVJGjmYPi" }, new Credential { Key = "N2", Cred = "" }}
            },
            new Student
            {
                Name = "studen2",
                Credentials = new List<Credential> {new Credential {Key = "N1", Cred = "PT7CpnUcG7DIrJTxN8CcqoHAyTbNNST3DzGLpGQUHF6/jyooYKW1puXb/a+WX2M8" }, new Credential { Key = "N2", Cred = "" }}
            },
        };

        var filterList = lstStudents.SelectMany(x => x.Credentials.Select(y => 
                new Data
                {
                    StudentName = x.Name, 
                    CredentialKey = y.Key,
                    UserName = utilities.DecryptStringFromBase64String(y.Cred, key).Before(":::") , 
                    Password = utilities.DecryptStringFromBase64String(y.Cred, key).After(":::")
                }))
            .Where(d => d.CredentialKey == "N1")
            .ToList();

配套班级，

public class Data
{
    public string StudentName { get; set; }
    public string CredentialKey { get; set; }
    public string UserName { get; set; }
    public string Password { get; set; }
}

public class Student
{
    public string Name { get; set; }
    public List<Credential> Credentials { get; set; }
}

public class Credential
{
    public string Key { get; set; }
    public string Cred { get; set; }
}

在上面的代码中，我调用DecryptStringFromBase64String方法 2 次，我只想调用它一次并准备我的数据 class，该怎么做？ 谢谢！

Answer 1

如果您调整查询以使用实际的 LINQ 语言语法（而不是扩展方法），这将更容易，因为您可以使用let ：

var filterList = (
    from x in lstStudents
    from y in x.Credentials
    let s = utilities.DecryptStringFromBase64String(y.Cred, key)
    let d = new Data
    {
        StudentName = x.Name,
        CredentialKey = y.Key,
        UserName = s.Before(":::"),
        Password = s.After(":::")
    }
    where d.CredentialKey == "N1"
    select d
).ToList();

或者更快地做where ，以避免一些计算/分配：

var filterList = (
    from x in lstStudents
    from y in x.Credentials
    where y.Key == "N1"
    let s = utilities.DecryptStringFromBase64String(y.Cred, key)
    let d = new Data
    {
        StudentName = x.Name,
        CredentialKey = y.Key,
        UserName = s.Before(":::"),
        Password = s.After(":::")
    }
    select d
).ToList();

就我个人而言，我也会更改所有x 、 y 、 s 、 d以使其更有意义，但我现在让它们保持不变，因此您可以看到它是如何映射的。

正如@Sinatr 在评论中指出的那样，另一种方法是使用带有正文的 lambda：

var filterList = lstStudents.SelectMany(x => x.Credentials.Select(y => {
    var s = utilities.DecryptStringFromBase64String(y.Cred, key);
    return new Data {
        StudentName = x.Name, 
        CredentialKey = y.Key,
        UserName = s.Before(":::") , 
        Password = s.After(":::")
    };
}))
.Where(d => d.CredentialKey == "N1")
.ToList();