diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index 007ce53c713e..191262d26597 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -6507,4 +6507,33 @@ protected Schema getAdditionalProperties(Schema schema) {
         return ModelUtils.getAdditionalProperties(openAPI, schema);
     }
 
+    final protected static Pattern JSON_MIME_PATTERN = Pattern.compile("(?i)application\\/json(;.*)?");
+    final protected static Pattern JSON_VENDOR_MIME_PATTERN = Pattern.compile("(?i)application\\/vnd.(.*)+json(;.*)?");
+
+    /**
+     * Check if the given MIME is a JSON MIME.
+     * JSON MIME examples:
+     * application/json
+     * application/json; charset=UTF8
+     * APPLICATION/JSON
+     *
+     * @param mime MIME string
+     * @return true if the input matches the JSON MIME
+     */
+    protected static boolean isJsonMimeType(String mime) {
+        return mime != null && (JSON_MIME_PATTERN.matcher(mime).matches());
+    }
+
+    /**
+     * Check if the given MIME is a JSON Vendor MIME.
+     * JSON MIME examples:
+     * application/vnd.mycompany+json
+     * application/vnd.mycompany.resourceA.version1+json
+     *
+     * @param mime MIME string
+     * @return true if the input matches the JSON vendor MIME
+     */
+    protected static boolean isJsonVendorMimeType(String mime) {
+        return mime != null && JSON_VENDOR_MIME_PATTERN.matcher(mime).matches();
+    }
 }
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 7941bb05f4c8..a59c2175f35c 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
@@ -657,6 +657,19 @@ public Map<String, Object> postProcessOperationsWithModels(Map<String, Object> o
                         }
                     }
 
+
+                    // check if the payload is json and set x-is-json accordingly
+                    if (operation.consumes != null) {
+                        for (Map<String, String> consume : operation.consumes) {
+                            if (consume.containsKey("mediaType")) {
+                                if (isJsonMimeType(consume.get("mediaType"))) {
+                                    operation.vendorExtensions.put("x-is-json", true);
+                                    break;
+                                }
+                            }
+                        }
+                    }
+
                     if (operation.examples != null) {
                         for (Map<String, String> example : operation.examples) {
                             for (Map.Entry<String, String> entry : example.entrySet()) {
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellHttpClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellHttpClientCodegen.java
index 00fc4e981756..b7dbc6ec53a4 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellHttpClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellHttpClientCodegen.java
@@ -1239,10 +1239,6 @@ public String escapeIdentifier(String prefix, String name) {
         return name;
     }
 
-    static boolean isJsonMimeType(String mime) {
-        return mime != null && JSON_MIME_PATTERN.matcher(mime).matches();
-    }
-
     static boolean isWildcardMimeType(String mime) {
         return mime != null && mime.equals("*/*");
     }
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java
index 664a65ffb472..a5cb4bc28dac 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java
@@ -926,29 +926,7 @@ public void forceSerializationLibrary(String serializationLibrary) {
         setSerializationLibrary(serializationLibrary);
     }
 
-    final private static Pattern JSON_MIME_PATTERN = Pattern.compile("(?i)application\\/json(;.*)?");
-    final private static Pattern JSON_VENDOR_MIME_PATTERN = Pattern.compile("(?i)application\\/vnd.(.*)+json(;.*)?");
 
-    /**
-     * Check if the given MIME is a JSON MIME.
-     * JSON MIME examples:
-     * application/json
-     * application/json; charset=UTF8
-     * APPLICATION/JSON
-     */
-    static boolean isJsonMimeType(String mime) {
-        return mime != null && (JSON_MIME_PATTERN.matcher(mime).matches());
-    }
-
-    /**
-     * Check if the given MIME is a JSON Vendor MIME.
-     * JSON MIME examples:
-     * application/vnd.mycompany+json
-     * application/vnd.mycompany.resourceA.version1+json
-     */
-    static boolean isJsonVendorMimeType(String mime) {
-        return mime != null && JSON_VENDOR_MIME_PATTERN.matcher(mime).matches();
-    }
 
     @Override
     public Map<String, Object> postProcessSupportingFileData(Map<String, Object> objs) {
diff --git a/modules/openapi-generator/src/main/resources/csharp/api.mustache b/modules/openapi-generator/src/main/resources/csharp/api.mustache
index f57926ade1dd..99592122f31a 100644
--- a/modules/openapi-generator/src/main/resources/csharp/api.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp/api.mustache
@@ -251,7 +251,7 @@ namespace {{packageName}}.{{apiPackage}}
             if ({{paramName}} != null) localVarHeaderParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToString({{paramName}})); // header parameter
             {{/headerParams}}
             {{#formParams}}
-            if ({{paramName}} != null) {{#isFile}}localVarFileParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToFile("{{baseName}}", {{paramName}}));{{/isFile}}{{^isFile}}localVarFormParams.Add("{{baseName}}", this.Configuration.ApiClient.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter{{/isFile}}
+            if ({{paramName}} != null) {{#isFile}}localVarFileParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToFile("{{baseName}}", {{paramName}}));{{/isFile}}{{^isFile}}localVarFormParams.Add("{{baseName}}", {{#vendorExtensions.x-is-json}}{{paramName}}.ToJson(){{/vendorExtensions.x-is-json}}{{^vendorExtensions.x-is-json}}this.Configuration.ApiClient.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}}){{/vendorExtensions.x-is-json}}); // form parameter{{/isFile}}
             {{/formParams}}
             {{#bodyParam}}
             if ({{paramName}} != null && {{paramName}}.GetType() != typeof(byte[]))
@@ -398,7 +398,7 @@ namespace {{packageName}}.{{apiPackage}}
             if ({{paramName}} != null) localVarHeaderParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToString({{paramName}})); // header parameter
             {{/headerParams}}
             {{#formParams}}
-            if ({{paramName}} != null) {{#isFile}}localVarFileParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToFile("{{baseName}}", {{paramName}}));{{/isFile}}{{^isFile}}localVarFormParams.Add("{{baseName}}", this.Configuration.ApiClient.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}})); // form parameter{{/isFile}}
+            if ({{paramName}} != null) {{#isFile}}localVarFileParams.Add("{{baseName}}", this.Configuration.ApiClient.ParameterToFile("{{baseName}}", {{paramName}}));{{/isFile}}{{^isFile}}localVarFormParams.Add("{{baseName}}", {{#vendorExtensions.x-is-json}}{{paramName}}.ToJson(){{/vendorExtensions.x-is-json}}{{^vendorExtensions.x-is-json}}this.Configuration.ApiClient.{{#isPrimitiveType}}ParameterToString{{/isPrimitiveType}}{{^isPrimitiveType}}Serialize{{/isPrimitiveType}}({{paramName}}){{/vendorExtensions.x-is-json}}); // form parameter{{/isFile}}
             {{/formParams}}
             {{#bodyParam}}
             if ({{paramName}} != null && {{paramName}}.GetType() != typeof(byte[]))