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 a1da5cffcb88..efb84f429330 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
@@ -4190,7 +4190,8 @@ public CodegenParameter fromParameter(Parameter parameter, Set<String> imports)
             // https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#user-content-parameterexplode
             codegenParameter.isExplode = parameter.getExplode() == null ? false : parameter.getExplode();
 
-            // TODO revise collectionFormat
+            // TODO revise collectionFormat, default collection format in OAS 3 appears to multi at least for query parameters
+            // https://swagger.io/docs/specification/serialization/
             String collectionFormat = null;
             if (ModelUtils.isArraySchema(parameterSchema)) { // for array parameter
                 final ArraySchema arraySchema = (ArraySchema) parameterSchema;
@@ -4346,14 +4347,21 @@ public CodegenParameter fromParameter(Parameter parameter, Set<String> imports)
         if (codegenParameter.isQueryParam && codegenParameter.isDeepObject) {
             Schema schema = ModelUtils.getSchema(openAPI, codegenParameter.dataType);
             codegenParameter.items = fromProperty(codegenParameter.paramName, schema);
-            Map<String, Schema<?>> properties = schema.getProperties();
-            codegenParameter.items.vars =
-                    properties.entrySet().stream()
-                            .map(entry -> {
-                                CodegenProperty property = fromProperty(entry.getKey(), entry.getValue());
-                                property.baseName = codegenParameter.baseName + "[" + entry.getKey() + "]";
-                                return property;
-                            }).collect(Collectors.toList());
+            // TODO Check why schema is actually null for a schema of type object defined inline
+            // https://swagger.io/docs/specification/serialization/
+            if(schema != null) {
+                Map<String, Schema<?>> properties = schema.getProperties();
+                codegenParameter.items.vars =
+                        properties.entrySet().stream()
+                                .map(entry -> {
+                                    CodegenProperty property = fromProperty(entry.getKey(), entry.getValue());
+                                    property.baseName = codegenParameter.baseName + "[" + entry.getKey() + "]";
+                                    return property;
+                                }).collect(Collectors.toList());
+            }
+            else {
+                LOGGER.warn("No object schema found for deepObject parameter" + codegenParameter + " deepObject won't have specific properties");
+            }
         }
 
         // set the parameter example value
diff --git a/modules/openapi-generator/src/main/resources/csharp-netcore/ClientUtils.mustache b/modules/openapi-generator/src/main/resources/csharp-netcore/ClientUtils.mustache
index e5fb0565fc9c..dd9a5f90fdcd 100644
--- a/modules/openapi-generator/src/main/resources/csharp-netcore/ClientUtils.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp-netcore/ClientUtils.mustache
@@ -63,6 +63,21 @@ namespace {{packageName}}.Client
                     parameters.Add(name, ParameterToString(item));
                 }
             }
+            else if (value is IDictionary dictionary)
+            {
+                if(collectionFormat == "deepObject") {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(name + "[" + entry.Key + "]", ParameterToString(entry.Value));
+                    }
+                }
+                else {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(entry.Key.ToString(), ParameterToString(entry.Value));
+                    }
+                }
+            }
             else
             {
                 parameters.Add(name, ParameterToString(value));
diff --git a/modules/openapi-generator/src/main/resources/csharp-netcore/api.mustache b/modules/openapi-generator/src/main/resources/csharp-netcore/api.mustache
index 0825406d5530..40d3bfd6596f 100644
--- a/modules/openapi-generator/src/main/resources/csharp-netcore/api.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp-netcore/api.mustache
@@ -280,7 +280,17 @@ namespace {{packageName}}.{{apiPackage}}
             {{/pathParams}}
             {{#queryParams}}
             {{#required}}
+            {{#isDeepObject}}
+            {{#items.vars}}
+            localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("{{#collectionFormat}}{{collectionFormat}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}.{{name}}));
+            {{/items.vars}}
+            {{^items}}
+            localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("deepObject", "{{baseName}}", {{paramName}}));
+            {{/items}}
+            {{/isDeepObject}}
+            {{^isDeepObject}}
             localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("{{#collectionFormat}}{{collectionFormat}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}));
+            {{/isDeepObject}}
             {{/required}}
             {{^required}}
             if ({{paramName}} != null)
@@ -292,6 +302,9 @@ namespace {{packageName}}.{{apiPackage}}
                     localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("{{#collectionFormat}}{{collectionFormat}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}.{{name}}));
                 }
                     {{/items.vars}}
+                {{^items}}
+                localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("deepObject", "{{baseName}}", {{paramName}}));
+                {{/items}}
                 {{/isDeepObject}}
                 {{^isDeepObject}}
                 localVarRequestOptions.QueryParameters.Add({{packageName}}.Client.ClientUtils.ParameterToMultiMap("{{#collectionFormat}}{{collectionFormat}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}));
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Client/ClientUtils.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Client/ClientUtils.cs
index 844e7e9a8f4b..2b6e52d60654 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Client/ClientUtils.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Client/ClientUtils.cs
@@ -67,6 +67,21 @@ public static Multimap<string, string> ParameterToMultiMap(string collectionForm
                     parameters.Add(name, ParameterToString(item));
                 }
             }
+            else if (value is IDictionary dictionary)
+            {
+                if(collectionFormat == "deepObject") {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(name + "[" + entry.Key + "]", ParameterToString(entry.Value));
+                    }
+                }
+                else {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(entry.Key.ToString(), ParameterToString(entry.Value));
+                    }
+                }
+            }
             else
             {
                 parameters.Add(name, ParameterToString(value));
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Client/ClientUtils.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Client/ClientUtils.cs
index 844e7e9a8f4b..2b6e52d60654 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Client/ClientUtils.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Client/ClientUtils.cs
@@ -67,6 +67,21 @@ public static Multimap<string, string> ParameterToMultiMap(string collectionForm
                     parameters.Add(name, ParameterToString(item));
                 }
             }
+            else if (value is IDictionary dictionary)
+            {
+                if(collectionFormat == "deepObject") {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(name + "[" + entry.Key + "]", ParameterToString(entry.Value));
+                    }
+                }
+                else {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(entry.Key.ToString(), ParameterToString(entry.Value));
+                    }
+                }
+            }
             else
             {
                 parameters.Add(name, ParameterToString(value));
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Client/ClientUtils.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Client/ClientUtils.cs
index 844e7e9a8f4b..2b6e52d60654 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Client/ClientUtils.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Client/ClientUtils.cs
@@ -67,6 +67,21 @@ public static Multimap<string, string> ParameterToMultiMap(string collectionForm
                     parameters.Add(name, ParameterToString(item));
                 }
             }
+            else if (value is IDictionary dictionary)
+            {
+                if(collectionFormat == "deepObject") {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(name + "[" + entry.Key + "]", ParameterToString(entry.Value));
+                    }
+                }
+                else {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(entry.Key.ToString(), ParameterToString(entry.Value));
+                    }
+                }
+            }
             else
             {
                 parameters.Add(name, ParameterToString(value));
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Client/ClientUtils.cs b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Client/ClientUtils.cs
index 844e7e9a8f4b..2b6e52d60654 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Client/ClientUtils.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Client/ClientUtils.cs
@@ -67,6 +67,21 @@ public static Multimap<string, string> ParameterToMultiMap(string collectionForm
                     parameters.Add(name, ParameterToString(item));
                 }
             }
+            else if (value is IDictionary dictionary)
+            {
+                if(collectionFormat == "deepObject") {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(name + "[" + entry.Key + "]", ParameterToString(entry.Value));
+                    }
+                }
+                else {
+                    foreach (DictionaryEntry entry in dictionary)
+                    {
+                        parameters.Add(entry.Key.ToString(), ParameterToString(entry.Value));
+                    }
+                }
+            }
             else
             {
                 parameters.Add(name, ParameterToString(value));