dotnet/Documentation/compatibility/list_foreach-can-throw-exception-when-modifying-list-item.md

List<T>.ForEach can throw exception when modifying list item

Scope

Edge

Version Introduced

4.5

Source Analyzer Status

Available

Change Description

Beginning in .NET 4.5, a xref:System.Collections.Generic.List`1.ForEach(System.Action{`0}) enumerator will throw an xref:System.InvalidOperationException?displayProperty=name exception if an element in the calling collection is modified. Previously, this would not throw an exception but could lead to race conditions.

• Quirked
• Build-time break

Recommended Action

Ideally, code should be fixed to not modify lists while enumerating their elements because that is never a safe operation. To revert to the previous behavior, though, an app may target .NET 4.0.

Affected APIs

• M:System.Collections.Generic.List`1.ForEach(System.Action{`0})

Category

Core