Question

I have a class Project as

public class Project 
{   public int ProjectId { get; set; }
    public string ProjectName { get; set; }
    public string Customer { get; set; }
    public string Address{ get; set; }
}

and I have 3 lists

List<Project> lst1; List<Project> lst2; List<Project> lst3;

lst1 contains Person objects with ProjectId and ProjectName.

ProjectId =1, ProjectName = "X", Customer = null, Address = null

ProjectId =2, ProjectName = "Y", Customer = null, Address = null

lst2 contains Person objects with ProjectId and Customer

ProjectId =1,ProjectName = null, Customer = "c1", Address = null

ProjectId =2,ProjectName = null, Customer = "c2", Address = null
, and

lst3 contains Person objects with ProjectId and Address

ProjectId = 1, ProjectName = null, Customer =null, Address = "a1"

ProjectId = 2, ProjectName = null, Customer =null, Address = "a2".

Considering there are multiple such records in each list and ProjectId is Uniqe for each project, How can I merge/combine these list to get one list with merged objects

ProjectId=1, ProjectName="X", Customer="c1", address="a1"

ProjectId=2, ProjectName="Y", Customer="c2", address="a2"

I found thse links similar and tried with it but could not meet the results

Create a list from two object lists with linq

How to merge two lists using LINQ?

Thank You.

Answer 1

This could be done in a multi-step approach pretty simply. First, define a Func<Project, Project, Project> to handle the actual record merging. That is, you are defining a method with a signature equivalent to public Project SomeMethod(Project p1, Project p2). This method implements the merging logic you outlined above. Next, we concatenate the elements of the lists together before grouping them by ProjectId, using our merge delegate as the an aggregate function in the overload of GroupBy which accepts a result selector:

Func<Project, Project, Project> mergeFunc = (p1,p2) => new Project
    {
        ProjectId = p1.ProjectId,
        ProjectName = p1.ProjectName == null ? p2.ProjectName : p1.ProjectName,
        Customer = p1.Customer == null ? p2.Customer : p1.Customer,
        Address = p1.Address == null ? p2.Address : p1.Address    
    };

var output = lst1.Concat(lst2).Concat(lst3)
                 .GroupBy(x => x.ProjectId, (k, g) => g.Aggregate(mergeFunc)); 

Here’s a quick and dirty test of the above logic along with output:

List<Project> lst1; List<Project> lst2; List<Project> lst3;
lst1 = new List<Project> 
    {
        new Project { ProjectId = 1, ProjectName = "P1" },
        new Project { ProjectId = 2, ProjectName = "P2" },
        new Project { ProjectId = 3, ProjectName = "P3" }
    };
lst2 = new List<Project>
    {
        new Project { ProjectId = 1, Customer = "Cust1"},
        new Project { ProjectId = 2, Customer = "Cust2"},
        new Project { ProjectId = 3, Customer = "Cust3"}
    };
lst3 = new List<Project>
    {
        new Project { ProjectId = 1, Address = "Add1"},
        new Project { ProjectId = 2, Address = "Add2"},
        new Project { ProjectId = 3, Address = "Add3"}
    };

Func<Project, Project, Project> mergeFunc = (p1,p2) => new Project
    {
        ProjectId = p1.ProjectId,
        ProjectName = p1.ProjectName == null ? p2.ProjectName : p1.ProjectName,
        Customer = p1.Customer == null ? p2.Customer : p1.Customer,
        Address = p1.Address == null ? p2.Address : p1.Address    
    };

var output = lst1
    .Concat(lst2)
    .Concat(lst3)
    .GroupBy(x => x.ProjectId, (k, g) => g.Aggregate(mergeFunc));

IEnumerable<bool> assertedCollection = output.Select((x, i) => 
    x.ProjectId == (i + 1) 
    && x.ProjectName == "P" + (i+1) 
    && x.Customer == "Cust" + (i+1) 
    && x.Address == "Add" + (i+1));

Debug.Assert(output.Count() == 3);  
Debug.Assert(assertedCollection.All(x => x == true));

— output —

IEnumerable<Project> (3 items)   
ProjectId ProjectName Customer Address 
1 P1 Cust1 Add1 
2 P2 Cust2 Add2 
3 P3 Cust3 Add3