diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java
index ffb716bdfda9..1de6d2f12004 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java
@@ -1263,6 +1263,19 @@ public void setParameterExampleValue(CodegenParameter codegenParameter) {
         }
     }
 
+    @Override
+    public void postProcessParameter(CodegenParameter parameter) {
+        super.postProcessParameter(parameter);
+
+        // ensure a method's parameters are marked as nullable when nullable or when nullReferences are enabled
+        // this is mostly needed for reference types used as a method's parameters
+        if (!parameter.required && (nullReferenceTypesFlag || nullableType.contains(parameter.dataType))) {
+            parameter.dataType = parameter.dataType.endsWith("?")
+                ? parameter.dataType
+                : parameter.dataType + "?";
+        }
+    }
+
     @Override
     public void postProcessFile(File file, String fileType) {
         if (file == null) {
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/csharp/CSharpOperationTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/csharp/CSharpOperationTest.java
new file mode 100644
index 000000000000..16ca55fac4cc
--- /dev/null
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/csharp/CSharpOperationTest.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2018 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ * Copyright 2018 SmartBear Software
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen;
+
+import com.google.common.collect.Sets;
+import com.samskivert.mustache.Mustache.Lambda;
+
+import io.swagger.parser.OpenAPIParser;
+import io.swagger.v3.oas.models.Components;
+import io.swagger.v3.oas.models.OpenAPI;
+import io.swagger.v3.oas.models.Operation;
+import io.swagger.v3.oas.models.PathItem;
+import io.swagger.v3.oas.models.headers.Header;
+import io.swagger.v3.oas.models.media.*;
+import io.swagger.v3.oas.models.parameters.QueryParameter;
+import io.swagger.v3.oas.models.parameters.RequestBody;
+import io.swagger.v3.oas.models.responses.ApiResponse;
+import io.swagger.v3.oas.models.responses.ApiResponses;
+import io.swagger.v3.oas.models.security.SecurityScheme;
+import io.swagger.v3.parser.core.models.ParseOptions;
+
+import org.openapitools.codegen.CodegenConstants;
+import org.openapitools.codegen.config.CodegenConfigurator;
+import org.openapitools.codegen.config.GlobalSettings;
+import org.openapitools.codegen.languages.AbstractCSharpCodegen;
+import org.openapitools.codegen.languages.AspNetCoreServerCodegen;
+import org.openapitools.codegen.languages.CSharpNetCoreClientCodegen;
+import org.openapitools.codegen.templating.mustache.CamelCaseLambda;
+import org.openapitools.codegen.templating.mustache.IndentedLambda;
+import org.openapitools.codegen.templating.mustache.LowercaseLambda;
+import org.openapitools.codegen.templating.mustache.TitlecaseLambda;
+import org.openapitools.codegen.templating.mustache.UppercaseLambda;
+import org.openapitools.codegen.utils.ModelUtils;
+import org.openapitools.codegen.utils.SemVer;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+import java.io.File;
+import java.nio.file.Files;
+import java.util.*;
+import java.util.logging.Logger;
+import java.util.stream.Collectors;
+
+import javax.accessibility.AccessibleAttributeSequence;
+
+import static org.testng.Assert.*;
+
+public class CSharpOperationTest {
+
+    @Test
+    public void assertMethodOptionalParameterDataType() {
+        assertEquals(getOperationOptionalParameterDataType(new AspNetCoreServerCodegen(), 2, false), "System.IO.Stream");
+        assertEquals(getOperationOptionalParameterDataType(new AspNetCoreServerCodegen(), 2, true), "System.IO.Stream?");
+        assertEquals(getOperationOptionalParameterDataType(new AspNetCoreServerCodegen(), 3, false), "System.IO.Stream");
+        assertEquals(getOperationOptionalParameterDataType(new AspNetCoreServerCodegen(), 3, true), "System.IO.Stream?");
+
+        assertEquals(getOperationOptionalParameterDataType(new CSharpNetCoreClientCodegen(), 2, false), "System.IO.Stream");
+        assertEquals(getOperationOptionalParameterDataType(new CSharpNetCoreClientCodegen(), 2, true), "System.IO.Stream?");
+        assertEquals(getOperationOptionalParameterDataType(new CSharpNetCoreClientCodegen(), 3, false), "System.IO.Stream");
+        assertEquals(getOperationOptionalParameterDataType(new CSharpNetCoreClientCodegen(), 3, true), "System.IO.Stream?");
+    }
+
+    public String getOperationOptionalParameterDataType(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.yaml");
+        codegen.setNullableReferenceTypes(nullableReferenceTypes);
+        codegen.setOpenAPI(openAPI);
+
+        final String path = "/pet/{petId}/uploadImage";
+        final Operation postOperation = openAPI.getPaths().get(path).getPost();
+        final CodegenOperation codegenOperation = codegen.fromOperation(path, "post", postOperation, null);
+
+        return codegenOperation.allParams.get(2).dataType;
+    }
+}