From 74ee70deeb31cb0e357a85d42ef7a05cf0bf85a6 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 19 Mar 2026 03:09:42 +0000
Subject: [PATCH 1/2] Initial plan


From f87a7d87515f887537e5b377a81c88b1a8533c61 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 19 Mar 2026 03:16:29 +0000
Subject: [PATCH 2/2] Fix ImmutableDictionary ArgumentException docs: clarify
 duplicate-key-same-value behavior

Co-authored-by: gewarren <24882762+gewarren@users.noreply.github.com>
---
 xml/System.Collections.Immutable/ImmutableDictionary.xml  | 8 ++++++++
 .../ImmutableDictionary`2+Builder.xml                     | 2 +-
 .../ImmutableDictionary`2.xml                             | 2 +-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/xml/System.Collections.Immutable/ImmutableDictionary.xml b/xml/System.Collections.Immutable/ImmutableDictionary.xml
index 74c6d61dd39..7de46261b7c 100644
--- a/xml/System.Collections.Immutable/ImmutableDictionary.xml
+++ b/xml/System.Collections.Immutable/ImmutableDictionary.xml
@@ -935,6 +935,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TKey,TValue&gt;">
@@ -1049,6 +1050,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents by using the specified key comparer.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TKey,TValue&gt;">
@@ -1126,6 +1128,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents by using the specified key and value comparers.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey&gt;">
@@ -1179,6 +1182,7 @@
         <summary>Constructs an immutable dictionary from an existing collection of elements, applying a transformation function to the source keys.</summary>
         <returns>The immutable dictionary that contains elements from <paramref name="source" />, with keys transformed by applying <paramref name="keySelector" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey&gt;">
@@ -1242,6 +1246,7 @@
         <summary>Constructs an immutable dictionary based on some transformation of a sequence.</summary>
         <returns>The immutable dictionary that contains elements from <paramref name="source" />, with keys transformed by applying <paramref name="keySelector" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1306,6 +1311,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1380,6 +1386,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents by using the specified key comparer.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1463,6 +1470,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents by using the specified key and value comparers.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
   </Members>
diff --git a/xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml b/xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml
index 44a410dd8ad..537183bca6d 100644
--- a/xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml
+++ b/xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml
@@ -188,7 +188,7 @@
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="key" /> is null.</exception>
-        <exception cref="T:System.ArgumentException">An element with the same key already exists in the dictionary.</exception>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
         <exception cref="T:System.NotSupportedException">The dictionary is read-only.</exception>
       </Docs>
     </Member>
diff --git a/xml/System.Collections.Immutable/ImmutableDictionary`2.xml b/xml/System.Collections.Immutable/ImmutableDictionary`2.xml
index 16f6906f140..8d3a98b4dc5 100644
--- a/xml/System.Collections.Immutable/ImmutableDictionary`2.xml
+++ b/xml/System.Collections.Immutable/ImmutableDictionary`2.xml
@@ -1051,7 +1051,7 @@ This example shows how to create a new immutable dictionary by adding and removi
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="key" /> is <see langword="null" />.</exception>
-        <exception cref="T:System.ArgumentException">An element with the same key already exists in the <see cref="T:System.Collections.Generic.IDictionary`2" />.</exception>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the <see cref="T:System.Collections.Generic.IDictionary`2" />.</exception>
         <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.IDictionary`2" /> is read-only.</exception>
       </Docs>
     </Member>