Fix: C# Code Generation generates method with return default(void) (#4826)

---------

Co-authored-by: Nikita Zimin <[email protected]>
Co-authored-by: Marko Lahma <[email protected]>

Author: 3 people

src/NSwag.CodeGeneration.CSharp.Tests/Snapshots/JIRA_OpenAPI.verified.txt

@@ -50345,7 +50345,7 @@ namespace MyNamespace
                         var status_ = (int)response_.StatusCode;
                         if (status_ == 200)
                         {
-                            return default(void);
+                            return;
                         }
                         else
                         if (status_ == 400)

src/NSwag.CodeGeneration.CSharp/Models/CSharpOperationModel.cs

@@ -115,7 +115,9 @@ public string MethodAccessModifier
         /// <summary>
         /// The default value of the result type, i.e. default(T) or default(T)! depending on whether NRT are enabled.
         /// </summary>
-        public string UnwrappedResultDefaultValue => $"default({UnwrappedResultType}){(_settings is CSharpClientGeneratorSettings { CSharpGeneratorSettings.GenerateNullableReferenceTypes: true } ? "!" : "")}";
+        public string UnwrappedResultDefaultValue => HasResult
+            ? $"default({UnwrappedResultType}){(_settings is CSharpClientGeneratorSettings { CSharpGeneratorSettings.GenerateNullableReferenceTypes: true } ? "!" : "")}"
+            : null;
 
         /// <summary>Gets or sets the synchronous type of the result.</summary>
         public string SyncResultType

src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.ProcessResponse.liquid

@@ -57,7 +57,7 @@ throw new {{ ExceptionClass }}<{{ response.Type }}>("{{ response.ExceptionDescri
 {%         if operation.WrapResponse -%}
 return new {{ ResponseClass }}<{{ operation.UnwrappedResultType }}>(status_, headers_, {{ operation.UnwrappedResultDefaultValue }});
 {%         else -%}
-return {{ operation.UnwrappedResultDefaultValue }};
+{%             if operation.HasResult %}return {{ operation.UnwrappedResultDefaultValue }};{% else %}return;{% endif %}
 {%         endif -%}
 {%     else -%}
 {%         if operation.WrapResponse -%}

src/NSwag.CodeGeneration.CSharp/Templates/Client.Class.liquid

@@ -413,7 +413,7 @@
 {%             if operation.WrapResponse and operation.UnwrappedResultType != "FileResponse" %}
                     return new {{ ResponseClass }}<{{ operation.UnwrappedResultType }}>(status_, headers_, {{ operation.UnwrappedResultDefaultValue }});
 {%             else -%}
-                    return {{ operation.UnwrappedResultDefaultValue }};
+                    {% if operation.HasResult %}return {{ operation.UnwrappedResultDefaultValue }};{% else %}return;{% endif %}
 {%             endif -%}
 {%         elsif operation.WrapResponse -%}
                     return new {{ ResponseClass }}(status_, headers_);
@@ -431,7 +431,7 @@
 {%                 if operation.WrapResponse and operation.UnwrappedResultType != "FileResponse" %}
                         return new {{ ResponseClass }}<{{ operation.UnwrappedResultType }}>(status_, headers_, {{ operation.UnwrappedResultDefaultValue }});
 {%                 else -%}
-                        return {{ operation.UnwrappedResultDefaultValue }};
+                        {% if operation.HasResult %}return {{ operation.UnwrappedResultDefaultValue }};{% else %}return;{% endif %}
 {%                 endif -%}
 {%             elsif operation.WrapResponse -%}
                         return new {{ ResponseClass }}(status_, headers_);