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Use Unsafe.As for generic cast #118684

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Use Unsafe.As for generic cast #118684

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4 changes: 2 additions & 2 deletions src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -116,7 +116,7 @@ private void AddRange(IEnumerable<KeyValuePair<TKey, TValue>> enumerable)
// back-compat with subclasses that may have overridden the enumerator behavior.
if (enumerable.GetType() == typeof(Dictionary<TKey, TValue>))
{
Dictionary<TKey, TValue> source = (Dictionary<TKey, TValue>)enumerable;
Dictionary<TKey, TValue> source = Unsafe.As<Dictionary<TKey, TValue>>(enumerable);

if (source.Count == 0)
{
Expand Down Expand Up @@ -161,7 +161,7 @@ private void AddRange(IEnumerable<KeyValuePair<TKey, TValue>> enumerable)
}
else if (enumerable.GetType() == typeof(List<KeyValuePair<TKey, TValue>>))
{
span = CollectionsMarshal.AsSpan((List<KeyValuePair<TKey, TValue>>)enumerable);
span = CollectionsMarshal.AsSpan(Unsafe.As<List<KeyValuePair<TKey, TValue>>>(enumerable));
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@stephentoub stephentoub Aug 13, 2025 •
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Thanks, but I do not want to introduce unsafe code for things like this. If it's material, the cast should be elided by the JIT, if it's not already.

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@EgorBo The diffs show the cast is not elided.

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Thanks, but I do not want to introduce unsafe code for things like this.

We already use the unsafe method CollectionsMarshal.AsSpan here.

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Thanks, but I do not want to introduce unsafe code for things like this.

We already use the unsafe method CollectionsMarshal.AsSpan here.

That doesn't mean we need to add more. They're also "unsafe" in very different ways. Unsafe.As lets you completely break type safety. AsSpan is on the marshal class only because there's a potential for someone to make changes that get lost if they get the span, then edit the list in a way that causes it to grow, and then edit the span; it's not a memory safety issue.

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@EgorBo The diffs show the cast is not elided.

Yes, this is a known issue, I can't find the exact github issue, but I do remember it exists. Basically, the same repro I mentioned here: #118655 (comment)
I'll try to fix it in .NET 11.0. In case if you're looking for an inspiration on what to contribute to dotnet/runtime repo, anything that replaces unsafe code with safe (ideally, with zero overhead) would be more than appreciated 🙂

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For reference, this is the issue:

}
else
{
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