Question

I have an IEnumerable collection of classes, and a second IEnumerable collection, containing a function that returns the same. For example:

IEnumerable<MyClass1> list1;
IEnumerable<MyClass2> list2;

MyClass2 contains an internal collection of MyClass1, and has a function as follows:

Public MyClass2
{
    private IEnumerable<MyClass1> internalCollection;

    public IEnumerable<MyClass1> ReturnDuplicates(IEnumerable<MyClass1> duplicates
    {                        
        return internalCollection.Where(l => duplicates.Select(d => d.ID).ToString() == l.ID.ToString());
    }
    ...
}

I then have a function to compare the two:

// Function to return a flag indicating if the two lists contain 
// any of the same data
public bool CheckDuplicates(IEnumerable<MyClass1> col1, IEnumerable<MyClass2> col2)
{
    // The following check is wrong as it returns null
    IEnumerable<MyClass1> dupe =                
            col2.Where(w => w.ReturnDuplicates(col1).Count() != 0)
                        as IEnumerable<MyClass1>;

    return (dupe.Count() != 0);
}

The above statement in CheckDuplicates() returns null. However, in the immediate window:

col2.First().ReturnDuplicates(col1).Count()

Returns 1

I think I know the cause of the problem – the statement is returning

IEnumerable<MyClass2>

so is this kind of check possible without a foreach statement?

Answer 1

Why are you casting as IEnumerable<MyClass1> ? The type of dupe seems to be IEnumerable<MyClass2>. That’s why your variable is null.

You may try the following code :

return col2.Any(w => w.ReturnDuplicates(col1).Any())

.Any() avoids the complete enumeration needed by .Count(). It will stop enumeration as soon as a match is found.