了解递归 LINQ function

Question

I have the following table

ItemNumber | FirstId | SecondId | ClientId
1          | 14      | 16       | NULL
2          | 17      | 18       | 1233242323
3          | 14      | 18       | 1233242323
5          | 15      | 12       | NULL
6          | 14      | 8        | 324234252
7          | 19      | 14       | 324234252
8          | 18      | 19       | 324234252
9          | 20      | 18       | 324234252

With following Class

public class ClientObject
{
    public int ItemNumber { get; set; }
    public int FirstId { get; set; }
    public int SecondId { get; set; }
    public double ClientId { get; set; }

    public ClientObject()
    {

    }
}

Given the FirstId and ClientId, I want to return SecondId where no matching is found in the FirstId column. Eg: Starting with FirstId as 20 ItemNumber 9), I would like to return 8 (ItemNumber 6).

I am trying to use a recursive function approach but not sure if it is right or if there is a better way to approach this problem.

public ClientObject GetItemRecursive(int initialId, double client)
{
    var returnThis = databaseCtxt.TableToUse
        .Where(x => x.FirstId == initialId && x.ClientId == client)
        .AsEnumerable() // updated from suggestion 
        .Select(x => GetItemRecursive(x.SecondId, x.ClientId))
        .FirstOrDefault();
    return returnThis ;
}

I have tried to set this up locally but have not been able to as this is a very very tiny part of large project, so I created the smallest examples here.

Answer 1

You actually need to check if the recursive call returns a null meaning there's no match and you've got to the "leaf".

public ClientObject GetItemRecursive(int initialId, double client)
{
    var initial = databaseCtxt.TableToUse
        .FirstOrDefault(x => x.FirstId == initialId && x.ClientId == client);
    if(initial == null) return null; 
    var child = GetItemRecursive(initial.SecondId, client);
    return child ?? initial;
}

Or you could write it without recursion

public ClientObject GetItemRecursive(int initialId, double client)
{
    ClientObject parent = null;
    bool childFound = true;
    while(childFound)
    {
        var id = parent?.SecondId ?? initialId
        var child = databaseCtxt.TableToUse
            .FirstOrDefault(x => x.FirstId == id && x.ClientId == client);
        if(child == null) childFound = false;
        else parent = child;
    }

    return parent;
}

Answer 2

I would assume you are looking for something like this

public ClientObject GetItemRecursive(int initialId, double client)
{
    return Inner(initialId, client, null);

    ClientObject Inner(int id, double d, ClientObject acc )
    {
       var current = databaseCtxt.TableToUse
         .Where(x => x.FirstId == id&& x.ClientId == d)
         .FirstOrDefault();
       if(current == null) return acc;    
       return Inner(current.SecondId, current.ClientId, current);
    }
}

This code uses C# 7.0 feature local functions fro convenience.

So general approach is that you make have an accumulator ( acc ) which is updated on each iteration until you can't find continuation(request to db returns null), when you reach this point you return the accumulator through all nested calls.