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 85cc92c65e13..3cbecf71893c 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
@@ -568,6 +568,32 @@ public Map<String, Object> postProcessOperationsWithModels(Map<String, Object> o
                         }
                     }
 
+                    for (CodegenParameter parameter: operation.allParams) {
+                        CodegenModel model = null;
+                        for(Object modelHashMap: allModels) {
+                            CodegenModel codegenModel = ((HashMap<String, CodegenModel>) modelHashMap).get("model");
+                            if (codegenModel.getClassname().equals(parameter.dataType)) {
+                                model = codegenModel;
+                                break;
+                            }
+                        }
+
+                        if (model == null) {
+                            // Primitive data types all come already marked
+                            parameter.isNullable = true;
+                        } else {
+                            // Effectively mark enum models as enums and non-nullable
+                            if (model.isEnum) {
+                                parameter.isEnum = true;
+                                parameter.allowableValues = model.allowableValues;
+                                parameter.isPrimitiveType = true;
+                                parameter.isNullable = false;
+                            } else {
+                                parameter.isNullable = true;
+                            }
+                        }
+                    }
+
                     processOperation(operation);
                 }
             }
diff --git a/modules/openapi-generator/src/main/resources/csharp/api.mustache b/modules/openapi-generator/src/main/resources/csharp/api.mustache
index cdbb940d3f53..85aa372e54f2 100644
--- a/modules/openapi-generator/src/main/resources/csharp/api.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp/api.mustache
@@ -32,7 +32,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>{{#returnType}}{{returnType}}{{/returnType}}</returns>
-        {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}} {{operationId}} ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
+        {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}} {{operationId}} ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
 
         /// <summary>
         /// {{summary}}
@@ -43,7 +43,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>ApiResponse of {{#returnType}}{{returnType}}{{/returnType}}{{^returnType}}Object(void){{/returnType}}</returns>
-        ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}> {{operationId}}WithHttpInfo ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
+        ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}> {{operationId}}WithHttpInfo ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
         {{/operation}}
         #endregion Synchronous Operations
         {{#supportsAsync}}
@@ -58,7 +58,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>Task of {{#returnType}}{{returnType}}{{/returnType}}{{^returnType}}void{{/returnType}}</returns>
-        {{#returnType}}System.Threading.Tasks.Task<{{{returnType}}}>{{/returnType}}{{^returnType}}System.Threading.Tasks.Task{{/returnType}} {{operationId}}Async ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
+        {{#returnType}}System.Threading.Tasks.Task<{{{returnType}}}>{{/returnType}}{{^returnType}}System.Threading.Tasks.Task{{/returnType}} {{operationId}}Async ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
 
         /// <summary>
         /// {{summary}}
@@ -69,7 +69,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>Task of ApiResponse{{#returnType}} ({{returnType}}){{/returnType}}</returns>
-        System.Threading.Tasks.Task<ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}>> {{operationId}}AsyncWithHttpInfo ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
+        System.Threading.Tasks.Task<ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}>> {{operationId}}AsyncWithHttpInfo ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
         {{/operation}}
         #endregion Asynchronous Operations
         {{/supportsAsync}}
@@ -190,7 +190,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>{{#returnType}}{{returnType}}{{/returnType}}</returns>
-        public {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}} {{operationId}} ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
+        public {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}} {{operationId}} ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
         {
              {{#returnType}}ApiResponse<{{{returnType}}}> localVarResponse = {{operationId}}WithHttpInfo({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
              return localVarResponse.Data;{{/returnType}}{{^returnType}}{{operationId}}WithHttpInfo({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}});{{/returnType}}
@@ -202,7 +202,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>ApiResponse of {{#returnType}}{{returnType}}{{/returnType}}{{^returnType}}Object(void){{/returnType}}</returns>
-        public ApiResponse<{{#returnType}} {{{returnType}}} {{/returnType}}{{^returnType}}Object{{/returnType}}> {{operationId}}WithHttpInfo ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
+        public ApiResponse<{{#returnType}} {{{returnType}}} {{/returnType}}{{^returnType}}Object{{/returnType}}> {{operationId}}WithHttpInfo ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
         {
             {{#allParams}}
             {{#required}}
@@ -325,7 +325,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>Task of {{#returnType}}{{returnType}}{{/returnType}}{{^returnType}}void{{/returnType}}</returns>
-        {{#returnType}}public async System.Threading.Tasks.Task<{{{returnType}}}>{{/returnType}}{{^returnType}}public async System.Threading.Tasks.Task{{/returnType}} {{operationId}}Async ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
+        {{#returnType}}public async System.Threading.Tasks.Task<{{{returnType}}}>{{/returnType}}{{^returnType}}public async System.Threading.Tasks.Task{{/returnType}} {{operationId}}Async ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
         {
              {{#returnType}}ApiResponse<{{{returnType}}}> localVarResponse = await {{operationId}}AsyncWithHttpInfo({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}});
              return localVarResponse.Data;{{/returnType}}{{^returnType}}await {{operationId}}AsyncWithHttpInfo({{#allParams}}{{paramName}}{{#hasMore}}, {{/hasMore}}{{/allParams}});{{/returnType}}
@@ -338,7 +338,7 @@ namespace {{packageName}}.{{apiPackage}}
         /// <exception cref="{{packageName}}.Client.ApiException">Thrown when fails to make API call</exception>
         {{#allParams}}/// <param name="{{paramName}}">{{description}}{{^required}} (optional{{#defaultValue}}, default to {{.}}{{/defaultValue}}){{/required}}</param>
         {{/allParams}}/// <returns>Task of ApiResponse{{#returnType}} ({{returnType}}){{/returnType}}</returns>
-        public async System.Threading.Tasks.Task<ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}>> {{operationId}}AsyncWithHttpInfo ({{#allParams}}{{{dataType}}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
+        public async System.Threading.Tasks.Task<ApiResponse<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Object{{/returnType}}>> {{operationId}}AsyncWithHttpInfo ({{#allParams}}{{{dataType}}}{{^required}}{{^isNullable}}?{{/isNullable}}{{/required}} {{paramName}}{{^required}}{{#optionalMethodArgument}} = null{{/optionalMethodArgument}}{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}})
         {
             {{#allParams}}
             {{#required}}
diff --git a/samples/client/petstore/csharp/OpenAPIClientNet35/.openapi-generator/VERSION b/samples/client/petstore/csharp/OpenAPIClientNet35/.openapi-generator/VERSION
index e24c1f857e01..d077ffb477a4 100644
--- a/samples/client/petstore/csharp/OpenAPIClientNet35/.openapi-generator/VERSION
+++ b/samples/client/petstore/csharp/OpenAPIClientNet35/.openapi-generator/VERSION
@@ -1 +1 @@
-3.3.3-SNAPSHOT
\ No newline at end of file
+3.3.4-SNAPSHOT
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/OpenAPIClientNet40/.openapi-generator/VERSION b/samples/client/petstore/csharp/OpenAPIClientNet40/.openapi-generator/VERSION
index e24c1f857e01..d077ffb477a4 100644
--- a/samples/client/petstore/csharp/OpenAPIClientNet40/.openapi-generator/VERSION
+++ b/samples/client/petstore/csharp/OpenAPIClientNet40/.openapi-generator/VERSION
@@ -1 +1 @@
-3.3.3-SNAPSHOT
\ No newline at end of file
+3.3.4-SNAPSHOT
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/OpenAPIClientNetStandard/.openapi-generator/VERSION b/samples/client/petstore/csharp/OpenAPIClientNetStandard/.openapi-generator/VERSION
index e24c1f857e01..d077ffb477a4 100644
--- a/samples/client/petstore/csharp/OpenAPIClientNetStandard/.openapi-generator/VERSION
+++ b/samples/client/petstore/csharp/OpenAPIClientNetStandard/.openapi-generator/VERSION
@@ -1 +1 @@
-3.3.3-SNAPSHOT
\ No newline at end of file
+3.3.4-SNAPSHOT
\ No newline at end of file
diff --git a/samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged/.openapi-generator/VERSION b/samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged/.openapi-generator/VERSION
index e24c1f857e01..d077ffb477a4 100644
--- a/samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged/.openapi-generator/VERSION
+++ b/samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged/.openapi-generator/VERSION
@@ -1 +1 @@
-3.3.3-SNAPSHOT
\ No newline at end of file
+3.3.4-SNAPSHOT
\ No newline at end of file