Improved string first letter case change

Packages.props

@@ -31,6 +31,7 @@
     <PackageReference Update="System.Diagnostics.DiagnosticSource" Version="$(SystemVersion)" />
     <PackageReference Update="System.Formats.Asn1" Version="$(SystemVersion)" />
     <PackageReference Update="System.Management" Version="6.*" />
+    <PackageReference Update="System.Memory" Version="4.6.*" />
     <PackageReference Update="System.Reactive" Version="6.*" NoWarn="NU1608" />
     <PackageReference Update="System.Text.Json" Version="$(SystemVersion)" />
     <PackageReference Update="System.Text.RegularExpressions" Version="4.3.1" />

src/FSharp.Data.GraphQL.Client/Extensions.fs

@@ -11,14 +11,15 @@ open System.Security.Cryptography
 /// Extensions for types used by the GraphQL client library.
 [<AutoOpen>]
 module internal Extensions =
+
+    let private changeFirstChar mapping (s : string) =
+        let span = s.AsSpan()
+        let c = mapping span[0]
+        if c = span[0] then s else string c + span.Slice(1).ToString()
+
     type String with
         /// Returns the input string with the first character in upper case.
-        member this.FirstCharUpper() =
-            this.Substring(0, 1).ToUpperInvariant() + this.Substring(1)
-
-        /// Returns the input string with the first character in lower case.
-        member this.FirstCharLower() =
-            this.Substring(0, 1).ToLowerInvariant() + this.Substring(1)
+        member this.FirstCharUpper () = changeFirstChar Char.ToUpperInvariant this
 
         member this.MD5Hash() =
             Encoding.UTF8.GetBytes(this)

src/FSharp.Data.GraphQL.Client/FSharp.Data.GraphQL.Client.fsproj

@@ -69,6 +69,8 @@
       <ExcludeAssets>runtime</ExcludeAssets>
     </PackageReference>
     <PackageReference Include="Microsoft.Extensions.Http" />
+    <PackageReference Include="System.Memory" />
+    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
 
   <ItemGroup>

src/FSharp.Data.GraphQL.Client/GraphQLClient.fs

@@ -8,6 +8,7 @@ open System.Collections.Generic
 open System.Collections.Immutable
 open System.Net.Http
 open System.Text
+open System.Text.Json
 open System.Threading
 open System.Threading.Tasks
 
@@ -131,7 +132,7 @@ module GraphQLClient =
                 | :? IDictionary<string, obj> as x ->
                     x
                     |> Seq.collect (fun kvp ->
-                        tryMapFileVariable (name + "." + (kvp.Key.FirstCharLower ()), kvp.Value)
+                        tryMapFileVariable (name + "." + JsonNamingPolicy.CamelCase.ConvertName kvp.Key, kvp.Value)
                         |> Option.defaultValue [||])
                     |> Array.ofSeq
                     |> Some

src/FSharp.Data.GraphQL.Client/Serialization.fs

@@ -8,6 +8,7 @@ open System.Collections.Generic
 open System.Diagnostics
 open System.Globalization
 open System.Reflection
+open System.Text.Json
 open Microsoft.FSharp.Reflection
 open FSharp.Data.GraphQL
 open FSharp.Data.GraphQL.Client.ReflectionPatterns
@@ -176,7 +177,7 @@ module Serialization =
                 | :? Upload as u -> JsonValue.String u.Name
                 | :? IDictionary<string, obj> as items ->
                     items
-                    |> Seq.map (fun (KeyValue (k, v)) -> k.FirstCharLower(), toJsonValue v)
+                    |> Seq.map (fun (KeyValue (k, v)) -> JsonNamingPolicy.CamelCase.ConvertName k, toJsonValue v)
                     |> Seq.toArray
                     |> JsonValue.Record
                 | EnumerableValue items ->
@@ -187,7 +188,7 @@ module Serialization =
                 | EnumValue x -> JsonValue.String x
                 | _ ->
                     let props = t.GetProperties(BindingFlags.Public ||| BindingFlags.Instance)
-                    let items = props |> Array.map (fun p -> (p.Name.FirstCharLower(), p.GetValue(x) |> toJsonValue))
+                    let items = props |> Array.map (fun p -> (JsonNamingPolicy.CamelCase.ConvertName p.Name, p.GetValue(x) |> toJsonValue))
                     JsonValue.Record items)
 
     let serializeRecord (x : obj) =