对实体框架具有副作用的嵌套 foreach 循环

Question

I have problem with nested foreach loops because of following example:

var someList = new List<SimpleC>();

simpleA = database.SimpleA.ToList();
simpleB = database.SimpleB.ToList();

foreach (var a in simpleA)
{
    foreach (var b in simpleB)
    {
        var c = new simpleC();
        c.simpleAId = a.Id;
        c.simpleAName = a.Name;
        c.simpleBName = b.Name;

        someList.Add(c);
    }
}

return someList;

The problem: imagine first iteration it goes to first foreach loop then to second foreach loop it initiates new object, maps values and adds newly initiated object to new list, but when second iteration comes instead of going back to first foreach loop it continues with second foreach loop and it loops through second until it finishes, I understand that's how C# works, but I need side effect for it to continue to first foreach loop to get new values.

Answer 1

I think, if I'm understanding your question, is that you don't want it to loop through in a nested fashion, but only loop once. To do that you'd do it like this.

for(int i = 0; i < simpleA.Count; i++)
{
    var c = new simpleC();
        c.simpleAId = simpleA[i].Name;
        c.simpleAName = simpleA[i].Name;
        c.simpleBName = simpleB[i].Name;
    someList.Add(c);
}

Answer 2

You would be much better served by not trying to iterate this manually, establishing a relationship and querying it as a join set of entities:

var results = database.SimpleA 
   .Join(database.SimpleB, 
      a => a.ID,       
      b => b.Post_ID,   /
      (a, b) => new { colA = a, colB = b }) 
   .Select new SimpleA(){
      Property1 = a.Col1,
      Property2 = b.Col3,
      Property4 = a.Col2
   }).ToList(); 

This is only mostly functional as a starting point. You might need a left outer join instead.

Another option, if A and B have a fk constraint in the database, then you may use Navigation properties instead of a join.

If for whatever reason, your loop is required we can try and breakdown the issue:

foreach (var a in simpleA)
{
    foreach (var b in simpleB)  // There is no relationship here!
    {
        var c = new simpleC();
        c.simpleAId = a.Id;
        c.simpleAName = a.Name;
        c.simpleBName = b.Name;

        someList.Add(c);
    }
}

You could try using the index, but that is very likely to cause issues.

Instead:

foreach (var a in simpleA)
{
    var matchedB = simpleB.Where(x => x.Id == a.Id).FirstOrDefault();

    var c = new simpleC();
    c.simpleAId = a.Id;
    c.simpleAName = a.Name;
    c.simpleBName = matchedB.Name;

    someList.Add(c);

}