how I can limit the call to only one time for method "utilities.DecryptStringFromBase64String"

Question

I am getting credential data from one of web service call and while decrypt it, the format is "username:::password". I have other class as well and finally I am creating Data class like below,

 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();

Supporting classes,

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; }
}

In above code I am calling method DecryptStringFromBase64String 2 times, I would like to call it only one time and prepare my data class, how to do this? Thanks!

Answer 1

This will be easier if you tweak the query to use the actual LINQ language syntax (rather than the extension methods), as then you can use 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();

or to do the where sooner, to avoid some calculations / allocations:

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();

Personally I'd also change all the x , y , s , d to be more meaningful, but I've left them the same for now, so you can see how it maps.

An alternative would be, as noted by @Sinatr in the comments, to use a lambda with body:

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();