C#: does ‘throw’ exit the current function

string is an alias in C# for System.String.
So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, it's generally recommended to use string any time you're referring to an object.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

string greet = String.Format("Hello {0}!", place);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

Ian Mercer posted a similar solution as this on Phil Haack's blog:

foreach (var item in Model.Select((value, i) => new { i, value }))
{
    var value = item.value;
    var index = item.i;
}

This gets you the item (item.value) and its index (item.i) by using this overload of LINQ's Select:

the second parameter of the function [inside Select] represents the index of the source element.

The new { i, value } is creating a new anonymous object.

Heap allocations can be avoided by using ValueTuple if you're using C# 7.0 or later:

foreach (var item in Model.Select((value, i) => ( value, i )))
{
    var value = item.value;
    var index = item.i;
}

You can also eliminate the item. by using automatic destructuring:

<ol>
foreach ((MyType value, Int32 i) in Model.Select((value, i) => ( value, i )))
{
    <li id="item_@i">@value</li>
}
</ol>