Add String.Create. (#424)

* Delete the redundant characters.

* Add String.Create.

Author: pengweiqhca

apiCount.include.md

@@ -1 +1 @@
-**API count: 642**
+**API count: 643**

api_list.include.md

@@ -704,6 +704,7 @@
  * `string[] Split(string, StringSplitOptions)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.split?view=net-10.0#system-string-split(system-string-system-stringsplitoptions))
  * `bool StartsWith(char)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.startswith?view=net-10.0#system-string-startswith(system-char))
  * `bool TryCopyTo(Span<char>)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.trycopyto?view=net-10.0)
+ * `string Create<TState>(int, TState, System.Buffers.SpanAction<char, TState>)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.create?view=net-10.0#system-string-create-1(system-int32-0-system-buffers-spanaction((system-char-0))))
  * `int GetHashCode(ReadOnlySpan<char>, StringComparison)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.gethashcode?view=net-10.0#system-string-gethashcode(system-readonlyspan((system-char))-system-stringcomparison))
  * `int GetHashCode(ReadOnlySpan<char>)` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.gethashcode?view=net-10.0#system-string-gethashcode(system-readonlyspan((system-char))))
  * `string Join(char, object?[])` [reference](https://learn.microsoft.com/en-us/dotnet/api/system.string.join?view=net-10.0#system-string-join(system-char-system-object()))

src/Polyfill/ReadOnlySpanAction.cs

@@ -0,0 +1,27 @@
+// <auto-generated />
+#pragma warning disable
+
+#if FeatureMemory
+#if NETFRAMEWORK || NETSTANDARD2_0 || NETCOREAPP2X
+#nullable enable
+
+namespace System.Buffers;
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+#if PolyUseEmbeddedAttribute
+[global::Microsoft.CodeAnalysis.EmbeddedAttribute]
+#endif
+// Encapsulates a method that receives a read-only span of objects of type T and a state object of type TArg.
+// https://learn.microsoft.com/en-us/dotnet/api/system.buffers.readonlyspanaction-2?view=net-10.0
+#if PolyPublic
+public
+#endif
+delegate void ReadOnlySpanAction<T, in TArg>(ReadOnlySpan<T> span, TArg arg);
+#else
+using System.Buffers;
+using System.Runtime.CompilerServices;
+[assembly: TypeForwardedTo(typeof(ReadOnlySpanAction<,>))]
+#endif
+#endif

src/Polyfill/SpanAction.cs

@@ -0,0 +1,27 @@
+// <auto-generated />
+#pragma warning disable
+
+#if FeatureMemory
+#if NETFRAMEWORK || NETSTANDARD2_0 || NETCOREAPP2X
+#nullable enable
+
+namespace System.Buffers;
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+#if PolyUseEmbeddedAttribute
+[global::Microsoft.CodeAnalysis.EmbeddedAttribute]
+#endif
+// Encapsulates a method that receives a span of objects of type T and a state object of type TArg.
+// https://learn.microsoft.com/en-us/dotnet/api/system.buffers.spanaction-2?view=net-10.0
+#if PolyPublic
+public
+#endif
+delegate void SpanAction<T, in TArg>(Span<T> span, TArg arg);
+#else
+using System.Buffers;
+using System.Runtime.CompilerServices;
+[assembly: TypeForwardedTo(typeof(SpanAction<,>))]
+#endif
+#endif

src/Polyfill/StringPolyfill.cs

@@ -1,4 +1,5 @@
 // <auto-generated />
+
 #pragma warning disable
 
 #if !NET9_0_OR_GREATER
@@ -217,6 +218,43 @@ public static string Join(char separator, string?[] value, int startIndex, int c
         //Link: https://learn.microsoft.com/en-us/dotnet/api/system.string.join?view=net-10.0#system-string-join-1(system-char-system-collections-generic-ienumerable((-0)))
         public static string Join<T>(char separator, IEnumerable<T> values) =>
             string.Join(new(separator, 1), values);
+#endif
+#if NETFRAMEWORK || NETSTANDARD2_0 || NETCOREAPP2X
+#if FeatureMemory
+        /// <summary>
+        /// Creates a new string with a specific length and initializes it after creation by using the specified callback.
+        /// </summary>
+        //Link: https://learn.microsoft.com/en-us/dotnet/api/system.string.create?view=net-10.0#system-string-create-1(system-int32-0-system-buffers-spanaction((system-char-0)))
+        public static string Create<TState>(int length, TState state, System.Buffers.SpanAction<char, TState> action)
+        {
+#if AllowUnsafeBlocks
+            var str = new string('\0', length);
+
+            unsafe
+            {
+                fixed (char* strPtr = str)
+                {
+                    action(new Span<char>(strPtr, length), state);
+                }
+            }
+
+            return str;
+#else
+            var chars = System.Buffers.ArrayPool<char>.Shared.Rent(length);
+
+            try
+            {
+                action(chars.AsSpan(0, length), state);
+
+                return new string(chars, 0, length);
+            }
+            finally
+            {
+                System.Buffers.ArrayPool<char>.Shared.Return(chars);
+            }
+#endif
+        }
+#endif
 #endif
     }
 }

src/Tests/StringPolyfillTests.cs

@@ -13,4 +13,13 @@ public void Join()
         // ReSharper disable once RedundantCast
         Assert.AreEqual("bac", string.Join('a', (IEnumerable<string>) new List<string>{"b", "c"}));
     }
+
+    [Test]
+    public void Create() => Assert.AreEqual("abcde", string.Create(5, 'a', (span, state) =>
+    {
+        for (var i = 0; i < span.Length; i++)
+        {
+            span[i] = state++;
+        }
+    }));
 }