From ec280c90b0d43209f493c03c25f395f8042268aa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?David=20Cant=C3=BA?= <dacantu@microsoft.com>
Date: Tue, 23 Apr 2024 18:19:20 -0500
Subject: [PATCH] Add implicit operator ROS<string?> for StringValues

---
 .../ref/Microsoft.Extensions.Primitives.cs    |  5 +++
 .../src/StringValues.cs                       | 20 ++++++++++
 .../tests/StringValuesTests.cs                | 37 +++++++++++++++++++
 3 files changed, 62 insertions(+)

diff --git a/src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs b/src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs
index a5f5f188981cd6..114ccc94e4552c 100644
--- a/src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs
+++ b/src/libraries/Microsoft.Extensions.Primitives/ref/Microsoft.Extensions.Primitives.cs
@@ -167,6 +167,11 @@ public void Reset() { }
         public static bool operator ==(string?[]? left, Microsoft.Extensions.Primitives.StringValues right) { throw null; }
         public static implicit operator string?(Microsoft.Extensions.Primitives.StringValues values) { throw null; }
         public static implicit operator string?[]?(Microsoft.Extensions.Primitives.StringValues value) { throw null; }
+#if NET9_0_OR_GREATER
+#pragma warning disable CS3006 // Overloaded method 'StringValues.implicit operator ReadOnlySpan<string?>(in StringValues)' differing only in ref or out, or in array rank, is not CLS-compliant.
+        public static implicit operator System.ReadOnlySpan<string?> (in Microsoft.Extensions.Primitives.StringValues value) { throw null; }
+#pragma warning restore CS3006
+#endif
         public static implicit operator Microsoft.Extensions.Primitives.StringValues(string? value) { throw null; }
         public static implicit operator Microsoft.Extensions.Primitives.StringValues(string?[]? values) { throw null; }
         public static bool operator !=(Microsoft.Extensions.Primitives.StringValues left, Microsoft.Extensions.Primitives.StringValues right) { throw null; }
diff --git a/src/libraries/Microsoft.Extensions.Primitives/src/StringValues.cs b/src/libraries/Microsoft.Extensions.Primitives/src/StringValues.cs
index 45cfea152227dc..6878d45d035706 100644
--- a/src/libraries/Microsoft.Extensions.Primitives/src/StringValues.cs
+++ b/src/libraries/Microsoft.Extensions.Primitives/src/StringValues.cs
@@ -85,6 +85,26 @@ public static implicit operator StringValues(string?[]? values)
             return value.GetArrayValue();
         }
 
+#if NET9_0_OR_GREATER
+        /// <summary>
+        /// Defines an implicit conversion of a given <see cref="StringValues"/> to a string span.
+        /// </summary>
+        /// <param name="value">A <see cref="StringValues"/> to implicitly convert.</param>
+#pragma warning disable CS3006 // Overloaded method 'StringValues.implicit operator ReadOnlySpan<string?>(in StringValues)' differing only in ref or out, or in array rank, is not CLS-compliant.
+        public static implicit operator ReadOnlySpan<string?>(in StringValues value)
+#pragma warning restore CS3006
+        {
+            if (value._values is string)
+            {
+                return new ReadOnlySpan<string?>(in Unsafe.As<object, string?>(ref Unsafe.AsRef(in value._values)));
+            }
+            else
+            {
+                return new ReadOnlySpan<string?>((string?[]?)value._values);
+            }
+        }
+#endif
+
         /// <summary>
         /// Gets the number of <see cref="string"/> elements contained in this <see cref="StringValues" />.
         /// </summary>
diff --git a/src/libraries/Microsoft.Extensions.Primitives/tests/StringValuesTests.cs b/src/libraries/Microsoft.Extensions.Primitives/tests/StringValuesTests.cs
index 5a0a0719ccc574..32be480cf2150d 100644
--- a/src/libraries/Microsoft.Extensions.Primitives/tests/StringValuesTests.cs
+++ b/src/libraries/Microsoft.Extensions.Primitives/tests/StringValuesTests.cs
@@ -275,6 +275,43 @@ public void ImplicitStringArrayConverter_Works()
             Assert.Equal<string[]>(aStringArray, stringValues);
         }
 
+#if NETCOREAPP
+        [Fact]
+        public void ImplicitReadOnlySpanStringConverter_Works()
+        {
+            // from null string
+            StringValues stringValues = (string)null;
+            ReadOnlySpan<string> span = (ReadOnlySpan<string>)stringValues;
+            Assert.True(span.IsEmpty);
+            Assert.Equal((string[])stringValues, span);
+
+            // from null string[]
+            stringValues = (string[])null;
+            span = (ReadOnlySpan<string>)stringValues;
+            Assert.True(span.IsEmpty);
+            Assert.Equal((string[])stringValues, span);
+
+            // from string
+            string aString = "abc";
+            stringValues = (string)aString;
+            span = (ReadOnlySpan<string>)stringValues;
+            Assert.Equal(1, span.Length);
+            Assert.Equal(aString, span[0]);
+            Assert.Equal((string[])stringValues, span);
+
+            // from string[]
+            string bString = "bcd";
+            string[] aStringArray = new[] { aString, bString };
+            stringValues = (string[])aStringArray;
+            span = (ReadOnlySpan<string>)stringValues;
+            Assert.Equal(2, span.Length);
+            Assert.Equal(aString, span[0]);
+            Assert.Equal(bString, span[1]);
+            Assert.Equal(aStringArray, span);
+            Assert.Equal((string[])stringValues, span);
+        }
+#endif
+
         [Theory]
         [MemberData(nameof(DefaultOrNullStringValues))]
         [MemberData(nameof(EmptyStringValues))]