[abstract csharp] Ensure enum dataType is not nullable string

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

@@ -431,6 +431,22 @@ public Map<String, Object> postProcessAllModels(Map<String, Object> objs) {
         return processed;
     }
 
+    @Override
+    protected List<Map<String, Object>> buildEnumVars(List<Object> values, String dataType) {
+        List<Map<String, Object>> enumVars = super.buildEnumVars(values, dataType);
+
+        // this is needed for enumRefs like OuterEnum marked as nullable and also have string values
+        // keep isString true so that the index will be used as the enum value instead of a string
+        // this is inline with C# enums with string values
+        if ("string?".equals(dataType)){
+            enumVars.forEach((enumVar) -> {
+                enumVar.put("isString", true);
+            }); 
+        }
+
+        return enumVars;
+    }
+
     /**
      * C# differs from other languages in that Enums are not _true_ objects; enums are compiled to integral types.
      * So, in C#, an enum is considers more like a user-defined primitive.

modules/openapi-generator/src/test/java/org/openapitools/codegen/csharp/CSharpModelTest.java

@@ -26,14 +26,42 @@
 import org.openapitools.codegen.CodegenProperty;
 import org.openapitools.codegen.DefaultCodegen;
 import org.openapitools.codegen.TestUtils;
+import org.openapitools.codegen.languages.AbstractCSharpCodegen;
 import org.openapitools.codegen.languages.AspNetCoreServerCodegen;
+import org.openapitools.codegen.languages.CSharpNetCoreClientCodegen;
 import org.openapitools.codegen.languages.CSharpClientCodegen;
 import org.testng.Assert;
 import org.testng.annotations.Test;
 
 @SuppressWarnings("static-method")
 public class CSharpModelTest {
 
+    @Test
+    public void assertOuterEnumIsString() {
+        // this issue has not been found yet in version 2
+        // Assert.assertEquals(outerEnumVarsIsString(new AspNetCoreServerCodegen(), 2, false), true);
+        // Assert.assertEquals(outerEnumVarsIsString(new AspNetCoreServerCodegen(), 2, true), true);
+        Assert.assertEquals(outerEnumVarsIsString(new AspNetCoreServerCodegen(), 3, false), true);
+        Assert.assertEquals(outerEnumVarsIsString(new AspNetCoreServerCodegen(), 3, true), true);
+
+        // this issue has not been found yet in version 2
+        // Assert.assertEquals(outerEnumVarsIsString(new CSharpNetCoreClientCodegen(), 2, false), true);
+        // Assert.assertEquals(outerEnumVarsIsString(new CSharpNetCoreClientCodegen(), 2, true), true);
+        Assert.assertEquals(outerEnumVarsIsString(new CSharpNetCoreClientCodegen(), 3, false), true);
+        Assert.assertEquals(outerEnumVarsIsString(new CSharpNetCoreClientCodegen(), 3, true), true);
+    }
+
+    public boolean outerEnumVarsIsString(final AbstractCSharpCodegen codegen, final int openApiVersion, final Boolean nullableReferenceTypes){
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/" + Integer.toString(openApiVersion) + "_0/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml");
+        codegen.setNullableReferenceTypes(nullableReferenceTypes);
+        codegen.setOpenAPI(openAPI);
+        Schema schema = openAPI.getComponents().getSchemas().get("Enum_Test");
+        final CodegenModel generated = codegen.fromModel("OuterEnum", schema);
+
+        CodegenProperty cp0 = generated.getVars().get(0);
+        return cp0.isString;
+    }
+
     @Test(description = "convert a model with array property to default List<T>")
     public void arrayPropertyTest() {
         final Schema schema = getArrayTestSchema();