Loops, Lambdas, and Variables

[chanan]

This might be a known Razor issue, and since I haven't used Razor in ages I was caught off guard.

This does not work:

@for (int i = 0; i < items.Count; i++)
{
  <CarouselItem IsActive="@(SecondIndex == i)" src="@item[i].Source" alt="@items[i].Alt">
    <h5>@items[0].Header</h5>
  </CarouselItem>
}

You get an exception with the h5 line (the previous lines are fine):

Uncaught Error: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at System.Collections.Generic.List`1[T].get_Item (System.Int32 index) <0x1f27f48 + 0x00018> in <140509c0d46c421ca83585ed625d5577>:0 
  at Sample.Pages.Carousels+<>c__DisplayClass0_0.<BuildRenderTree>b__10 (Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder3) <0x1f3d190 + 0x00090> in <14d8d1e2ea4743fa889b8a5ddf494a58>:0 

The work around is to add a capture variable in the loop like so:

@for (int i = 0; i < items.Count; i++)
{
  Item item = items[i];
  <CarouselItem IsActive="@(SecondIndex == i)" src="@item.Source" alt="@item.Alt">
    <h5>@item.Header</h5>
  </CarouselItem>
}

Like I said not sure if this is fixable since it might be in Razor like that, but at the very least a better error message and adding this to the docs would be helpful.

[figloalds]

In your first block of code you have this line

<h5>@item[0].Header</h5>

which seems to reference an array item (as opposed to items) that is apparently not anywhere else aforementioned.

[dlr1]

try this. you would need to use local variable for it to work

@for (int i = 0; i < items.Count; i++)
{
var local_i = i;
  <CarouselItem IsActive="@(SecondIndex == local_i)" src="@item[local_i].Source" alt="@items[local_i].Alt">
    <h5>@item[0].Header</h5>
  </CarouselItem>
}

[chanan]

@felyperennan That was a typo when copying and pasting into the issue.
@dlr1 Thanks, but that is basically the same as my workaround :) Anyway, point being if it is something that can't be fixed because it is in Razor, I just think it should be noted in the docs, I happened to remember that issue from my C#/Razor days, but a newbie wouldnt know that.

[dlr1]

I opened a similar one aspnet/Blazor#665

[SteveSandersonMS]

This is a general C# question about how loops and lambda hoisting interact. The answer is that foreach is different from for, in that for conceptually defines a new loop variable for each iteration, whereas for does not (it's the same variable across all iterations). Please see C# docs for more about this.