How to Find Lookup field list using Programmatically

Question

How to find the list that a particular lookup column in a list is bound to, programatically?

Answer 1

The easiest way is to look in MSDN documentation http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfieldlookup.lookuplist.aspx

In the link you can find the below example which I guess is what you are looking for.

using System;
using Microsoft.SharePoint;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            using (SPSite siteCollection = new SPSite("http://localhost"))
            {
                using (SPWeb site = siteCollection.OpenWeb())
                {
                    SPList list = site.Lists["Pending Orders"];

                    foreach (SPField item in list.Fields)
                    {
                        if (item is SPFieldLookup)
                        {
                            SPFieldLookup field = (SPFieldLookup)item;

                            if (!String.IsNullOrEmpty(field.LookupList) && !String.IsNullOrEmpty(field.LookupField))
                            {
                                // Is this the primary or secondary field for a list relationship?
                                string strRelationship = field.IsRelationship ? "Primary":"Secondary";

                                // Print the display name of the field.
                                Console.WriteLine("\nField: {0} ({1} Field)", field.Title, strRelationship);

                                // Is this a secondary field in a list relationship?
                                if (field.IsDependentLookup)
                                {
                                    SPField primaryField = list.Fields[new Guid(field.PrimaryFieldId)];
                                    Console.WriteLine("Primary Field: {0}", primaryField.Title);
                                }

                                // Get the site where the target list is located.
                                using (SPWeb targetSite = siteCollection.AllWebs[field.LookupWebId])
                                {
                                    // Get the name of the list where this field gets information.
                                    SPList targetList = targetSite.Lists[new Guid(field.LookupList)];
                                    Console.WriteLine("Related list: {0}", targetList.Title);

                                    // Get the name of the field where this field gets information.
                                    SPField targetField = targetList.Fields.GetFieldByInternalName(field.LookupField);
                                    Console.WriteLine("Related field: {0}", targetField.Title);
                                }
                            }
                        }
                    }
                }
            }
            Console.Write("\nPress ENTER to continue...");
            Console.ReadLine();
        }
    }
}

Quick and DIRTY Pseudocode example:

function IsFieldInUse(splist, spfield, currentweb)
{
    foreach (field in currentweb.fields)
    {
        if (field is spfieldlookup)
        {
            if (field.lookuplist==splist && field.lookupfield==spfield) return true;
        }
    }

    foreach (list in currentweb.lists)
    {
        foreach (field in list.fields)
        {
            if (field is spfieldlookup)
            {
                if (field.lookuplist==splist && field.lookupfield==spfield) return true;
            }
        }
    }

    foreach (web in currentweb.webs)
    {
        if (IsFieldInUse(splist, spfield, web))
        {
            return true;
        }
    }
    return false;   
}

function IsFieldInUse(splist, spfield)
{
    return IsFieldInUse(splist, spfield, currentSite.rootweb)
}

Answer 2

You can use: var value = SP.FieldLookup.get_lookupList ()

I found the answer here: https://msdn.microsoft.com/en-us/library/office/jj245483.aspx