C# Pass lambda expression field into method and use field in linq query

Question

How can I pass a field (via lambda expression) into a method and then use that field as part of a linq query?

I would like to call the method something like

IDictionary<string, string> stuff = foo(items, otherItems, otherItems => otherItems.FieldToUse)

I'm not sure how I would write the method, but I would want to use it sort of like the code below. I know I could use generics and pass the field name (via a string) into the method, but even then I don't know how I would use it in the linq query as follows. Also, I like using lambda, since I could just rename the field anytime I choose.

private IDictionary<string, string> foo<TModel>(IEnumerable<string> items, IEnumerable<TModel> otherItems, object FieldToUse)
    {
        //this will return a list of key value pairs of rowIDs and equipment
        IDictionary<string, string> x = (from o in otherItems
                                         join i in items on o.FieldToUse.ToString() equals i //joining on the equipment assetcode
                                         select new { rowID = o.RowID, item = i }).ToDictionary(k => k.rowID.ToString(), v => v.item);
        return x;
    }

Clarification: FieldToUse is a property or field of TModel

Question is not very clear. FieldsToUse is a type in otherItems ? When you are passing otherItems, there is no need to pass the field. — Nagashree Hs
– Nagashree Hs, Commented Sep 1, 2016 at 15:26
I don't always want to call the method using the same property, even if I am using the same type for TModel. Sometimes I want to use one property and sometimes another property — TruthOf42
– TruthOf42, Commented Sep 1, 2016 at 15:31
I think you want to use a selector. An example would be LINQs OrderBy method where you would call .OrderBy(obj => obj.Property); is that what you are looking for? — mbrdev
– mbrdev, Commented Sep 1, 2016 at 15:47
Do you need this to work against LINQ to Entities/SQL? Or is it enough to work against normal objects? — StriplingWarrior
– StriplingWarrior, Commented Sep 1, 2016 at 15:54

robor · Accepted Answer · 2016-09-01 15:49:09Z

Use a Func Delegate

Change the last parameter in method foo to

Func<TModel, String> FieldToUse

and in the LINQ query call the function

FieldToUse(o)

Here is the entire method foo

private IDictionary<string, string> foo<TModel>(IEnumerable<string> items,
  IEnumerable<TModel> otherItems,
  Func<TModel, String> FieldToUse)
{
  //this will return a list of key value pairs of rowIDs and equipment
  IDictionary<string, string> x = (from o in otherItems
                                   join i in items on FieldToUse(o) equals i //joining on the equipment assetcode
                                   select new { rowID = o.RowID, item = i })
                                   .ToDictionary(k => k.rowID.ToString(), v => v.item);
  return x;
}

This is how you can use it

public void DoStuff()
{
  string[] items = { "abc", "def", "ghi" };
  List<Model> otherItems = new List<Model> { 
        new Model() { Field1 = "abc", Field2 = "xyz" }, 
        new Model() { Field1 = "abc", Field2 = "xyz" } };

  var result = foo<Model>(items, otherItems, a => a.Field2);
}

class Model 
{
  public string Field1 { get; set; }
  public string Field2 { get; set; }
}

You will have another problem though. The generic TModel does not have RowID. Perhaps provide a generic where constraint for TModel.

The code then becomes

 private IDictionary<string, string> foo<TModel>(IEnumerable<string> items,
  IEnumerable<TModel> otherItems,
  Func<TModel, String> FieldToUse) where TModel : BaseModel
{
  //this will return a list of key value pairs of rowIDs and equipment
  IDictionary<string, string> x = (from o in otherItems
                                   join i in items on FieldToUse(o) equals i //joining on the equipment assetcode
                                   select new { rowID = o.RowID, item = i })
                                   .ToDictionary(k => k.rowID.ToString(), v => v.item);
  return x;
}

class BaseModel
{
  public int RowID { get; set; }
}
class Model : BaseModel
{
  public string Field1 { get; set; }
  public string Field2 { get; set; }
}

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