[BUG][C#] Should use fully-qualified class name for models

Description

I have exactly the same issue as this: https://stackoverflow.com/questions/59692538/ambiguous-reference-error-in-openapi-generated-c-sharp-code

I have a model class called Environment and using the csharp-netcore generator this causes compile errors due to the ambiguous reference.

I could not see this listed as a bug for OpenAPI Generator.

There is an open issue for Swagger codegen https://github.com/swagger-api/swagger-codegen/issues/2630 as mentioned.

Error is:

Api\EnvironmentApi.cs(1053,173): error CS0104: 'Environment' is an ambiguous reference between 'Org.OpenAPITools.Model.Environment' and 'System.Environment'

openapi-generator version

6.6.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {
    "title": "CloudData",
    "version": "1.0"
  },
  "paths": {
    "/api/v1/environments": {
      "get": {
        "tags": [
          "Environment"
        ],
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Environment"
                  }
                }
              },
              "application/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Environment"
                  }
                }
              },
              "text/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Environment"
                  }
                }
              }
            }
          }
        }
      },
      "post": {
        "tags": [
          "Environment"
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            },
            "text/json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            },
            "application/*+json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              }
            }
          }
        }
      }
    },
    "/api/v1/environments/{id}": {
      "get": {
        "tags": [
          "Environment"
        ],
        "parameters": [
          {
            "name": "id",
            "in": "path",
            "required": true,
            "schema": {
              "type": "string",
              "format": "uuid"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/Environment"
                }
              }
            }
          }
        }
      },
      "put": {
        "tags": [
          "Environment"
        ],
        "parameters": [
          {
            "name": "id",
            "in": "path",
            "required": true,
            "schema": {
              "type": "string",
              "format": "uuid"
            }
          }
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            },
            "text/json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            },
            "application/*+json": {
              "schema": {
                "$ref": "#/components/schemas/Environment"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success"
          }
        }
      },
      "delete": {
        "tags": [
          "Environment"
        ],
        "parameters": [
          {
            "name": "id",
            "in": "path",
            "required": true,
            "schema": {
              "type": "string",
              "format": "uuid"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "Success"
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Environment": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string",
            "format": "uuid"
          },
          "name": {
            "type": "string",
            "nullable": true
          }
        },
        "additionalProperties": false
      }
    },
    "securitySchemes": {
      "Bearer": {
        "type": "apiKey",
        "description": "JWT Token issued for your account using Bearer scheme.",
        "name": "Authorization",
        "in": "header"
      }
    }
  },
  "security": [
    {
      "Bearer": [ ]
    }
  ]
}

Generation Details

npm i @openapitools/openapi-generator-cli
npx openapi-generator-cli generate -i swagger.json -g csharp-netcore -o output

Steps to reproduce

cd output
dotnet build

Related issues/PRs

https://stackoverflow.com/questions/59692538/ambiguous-reference-error-in-openapi-generated-c-sharp-code
https://github.com/swagger-api/swagger-codegen/issues/2630

Suggest a fix

C# generator should be changed to use fully qualified class names.

Start by generating the supplied OpenAPI declaration with the documented csharp-netcore command, then run dotnet build in the output directory. Inspect the generated Api/EnvironmentApi.cs references that produce the ambiguity; done means the generated project builds successfully when a model is named Environment.