Allow setting x-cSharpExistingType on schemas

to override default resolved type

src/NJsonSchema.CodeGeneration.CSharp.Tests/ResolveExistingTypeTests.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace NJsonSchema.CodeGeneration.CSharp.Tests;
+
+public class ResolveExistingTypeTests
+{
+    [Fact]
+    public async Task String_schema_can_use_existing_type()
+    {
+        var json = @"{
+            ""type"": ""object"",
+            ""properties"": {
+                ""ip"": {
+                    ""type"": ""string"",
+                    ""pattern"": ""^\\d{1,3}(\\.\\d{1,3}){3}$"",
+                    ""x-cSharpExistingType"": ""System.Net.IPAddress""
+                }
+            }
+        }";
+
+        var schema = await JsonSchema.FromJsonAsync(json);
+
+        // Act
+        var code = new CSharpGenerator(schema).GenerateFile("MyClass");
+
+        //// Act
+        Assert.Contains("public System.Net.IPAddress Ip { get; set; }", code);
+    }
+
+    [Fact]
+    public async Task String_schema_can_use_existing_type_in_definition()
+    {
+        var json = @"{
+            ""type"": ""object"",
+            ""properties"": {
+                ""ip"": { ""$ref"": ""#/definitions/ip"" }
+            },
+            ""definitions"": {
+                ""ip"": {
+                    ""type"": ""string"",
+                    ""pattern"": ""^\\d{1,3}(\\.\\d{1,3}){3}$"",
+                    ""x-cSharpExistingType"": ""System.Net.IPAddress""
+                }
+            }
+        }";
+
+        var schema = await JsonSchema.FromJsonAsync(json);
+
+        // Act
+        var code = new CSharpGenerator(schema).GenerateFile("MyClass");
+
+        //// Act
+        Assert.Contains("public System.Net.IPAddress Ip { get; set; }", code);
+    }
+}

src/NJsonSchema.CodeGeneration.CSharp/CSharpTypeResolver.cs

@@ -77,6 +77,12 @@ public string Resolve(JsonSchema schema, bool isNullable, string typeNameHint, b
 
             var markAsNullableType = Settings.GenerateNullableReferenceTypes && isNullable;
 
+            if (schema.ExtensionData != null &&
+                schema.ExtensionData.TryGetValue("x-cSharpExistingType", out var cSharpExistingType))
+            {
+                return cSharpExistingType + (markAsNullableType ? "?" : "");
+            }
+
             if (schema.ActualTypeSchema.IsAnyType &&
                 schema.ActualDiscriminator == null &&
                 schema.InheritedSchema == null && // not in inheritance hierarchy