How can foreach know the item type of a 2D array when it's doesn't implement IEnumerable T ?

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I can't seem to reconcile these two observations:

A 2D array (T[,]) can't be assigned to a variable of type IEnumerable T
In foreach(var item in (T[,])array2d), the compiler knows that var is T.

A 2D array (T[,]) can't be assigned to a variable of type IEnumerable T
T[,]
IEnumerable T
In foreach(var item in (T[,])array2d), the compiler knows that var is T.
foreach(var item in (T[,])array2d)
var
T
I thought foreach was just syntactic sugar that uses IEnumerable T .GetEnumerator() (in which case var is T), or IEnumerable.GetEnumerator() (in which case var is object).
foreach
IEnumerable T .GetEnumerator()
var
T
IEnumerable.GetEnumerator()
var
object
Here is the scenario that led me to this question. I have a method that takes an IEnumerable Foo .
IEnumerable Foo
int Average(IEnumerable Foo values)
{
// lots of code
foreach (var foo in values)
{
var bar = foo.GetBar();
// lots of code
}
// lots of code
return result;
}

Foo[] array1d = /**/;
Average(array1d);

int Average(IEnumerable Foo values)
{
// lots of code
foreach (var foo in values)
{
var bar = foo.GetBar();
// lots of code
}
// lots of code
return result;
}

Foo[] array1d = /**/;
Average(array1d);

I then wanted to pass a 2D array to this method, but it doesn't compile; the error is CS1503 Argument 1: cannot convert from 'Foo[*,*]' to 'System.Collections.Generic.IEnumerable Foo '.
CS1503 Argument 1: cannot convert from 'Foo[*,*]' to 'System.Collections.Generic.IEnumerable Foo '
Foo[,] array2d = /**/;
Average(array2d);

Foo[,] array2d = /**/;
Average(array2d);

However, I discovered that it does compile if I just add this overload. When I mouse over var foo, the compiler knows that it is a Foo, and is able to find Foo.GetBar().
does
var foo
Foo
Foo.GetBar()
void Average(Foo[,] values)
{
// lots of code
foreach (var foo in values)
{
var bar = foo.GetBar();
// lots of code
}
// lots of code
return result;
}

void Average(Foo[,] values)
{
// lots of code
foreach (var foo in values)
{
var bar = foo.GetBar();
// lots of code
}
// lots of code
return result;
}

Unfortunately, this duplicates my code. I could alternatively call Average(array2d.Cast Foo ());, but this adds an extra iterator, and I'm looking for performance. The iterator probably won't affect performance significantly, but it feels like there has to be a better way.
Average(array2d.Cast Foo ());

Tags: c#,multidimensional-array,foreach,var,c#-12.0Source of the question:

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