C# decimal default literal doesn't compile

Created by: wheezil

A field of type "number" with no format is mapped to decimal/Decimal types in various languages. However, the utility method ModelUtils.IsDouble() incorrectly reports true. This leads to cases where e.g. generated C# code creates fields of type decimal but tries to initialize them with default values that are double-formatted literals ("0.1D") instead of decimal-format literals ("0.1M"). Code does not compile.

Description

Bug in ModelUtils.isDoubleSchema makes C# codegen produce incorrect literals, and probably affects other languages.

openapi-generator version

3.3.2

OpenAPI declaration file content or url

Any schema with field declaration of number without format, and with a default value e.g.:

{
   "openapi": "3.0.0",
   "info": {
      "description": "blah",
      "version": "1.0.0",
      "title": "blah"
   },
   "paths": {
   },
   "components": {
      "requestBodies": {
      },
      "schemas": {
         "Part": {
            "properties": {
               "cost":{
                  "type": "number",
                  "default":"0.0"
               }
            }
         }
      }
   }
}

Command line used for generation

Specifically the csharp generator has problems, but probably any language that maps number to a "decimal" type will have difficulties because it cannot distinguish double from "other number"

generate -g csharp ...

Steps to reproduce

Run generation with csharp laugnage option. Note that generated Cost field is of type decimal:

public decimal? Cost { get; set; }

But default initializer doesn't compile:

        public Part(decimal? cost = 0.0D)
        {
            // use default value if no "cost" provided
            if (cost == null)
            {
                this.Cost = 0.0D;
            }

The literal would need to be 0.0M not 0.0D. This looks like a bug in AbstractCSharpCodegen, which formats the literal as 0.0D instead of 0.0M:

    public String toDefaultValue(Schema p) {
        ...
        } else if (ModelUtils.isNumberSchema(p)) {
            if (p.getDefault() != null) {
                if (ModelUtils.isFloatSchema(p)) { // float
                    return p.getDefault().toString() + "F";
                } else if (ModelUtils.isDoubleSchema(p)) { // double
                    return p.getDefault().toString() + "D";
                } else {
                    return p.getDefault().toString();
                }
            }

The method ModelUtils.isDoubleSchema(p) incorrectly returns true, making C# codegen produce the wrong default literal.

However... even if this were fixed, the C# codegen is still wrong, because it needs to add "M" to the decimal literal:

                    ...
                    return p.getDefault().toString() + "M";

Related issues/PRs

Suggest a fix/enhancement

In this method

    public static boolean isDoubleSchema(Schema schema) {
        if (schema instanceof NumberSchema) {
            return true;
        }
        if (SchemaTypeUtil.NUMBER_TYPE.equals(schema.getType())
                && SchemaTypeUtil.DOUBLE_FORMAT.equals(schema.getFormat())) { // format: double
            return true;
        }
        return false;
    }

These lines should be removed:

        if (schema instanceof NumberSchema) {
            return true;
        }

And in AbstractCSharpCodegen.toDefaultValue(Schema p), return the correct decimal literal in default case:

    public String toDefaultValue(Schema p) {
        ...
        } else if (ModelUtils.isNumberSchema(p)) {
                ...
                } else {
                    return p.getDefault().toString() + "M";
                }
            }