[BUG] C# CodeGen doesnt respect nullable value for custom mapping with timespan

[aswinfrancis91]

Description

I have below two properties

[Required]
public TimeSpan StartTime { get; set; }
public TimeSpan? EndTime { get; set; }

but since I can't generate SDK with same timespan field from the swagger json I added below line to startup class

c.MapType<TimeSpan>(() => new OpenApiSchema { Type = "string", Format = "time-span", Nullable = false });
c.MapType<TimeSpan?>(() => new OpenApiSchema { Type = "string", Format = "time-span", Nullable = true });

The json generated also looks good

"startTime": {
            "type": "string",
            "format": "time-span"
          },
 "endTime": {
            "type": "string",
            "format": "time-span",
            "nullable": true
          },

I then pass the below type mapping argument
--type-mappings time-span=TimeSpan

The problem here is that the non nullable fields comes fine in the SDK generated but the nullable field comes as non nullable itself. Below is the C# code that was generated

[DataMember(Name = "startTime", IsRequired = true, EmitDefaultValue = true)]
public TimeSpan StartTime { get; set; }

[DataMember(Name = "endTime", EmitDefaultValue = true)]
 public TimeSpan EndTime { get; set; }

openapi-generator version

6.2.1

Related issues/PRs

#4951

[aswinfrancis91]

Anyone looking into this?

[sharifahmad2061]

No solution yet.

[leonluc-dev]

I've looked into it since I ran into this issue myself. This doesn't just apply to TimeSpan but to any struct or value type not listed here:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

Lines 1992 to 2022 in e87c4ea

	protected void setTypeMapping() {
	typeMapping = new HashMap<>();
	typeMapping.put("string", "string");
	typeMapping.put("binary", "byte[]");
	typeMapping.put("ByteArray", "byte[]");
	typeMapping.put("boolean", "bool");
	typeMapping.put("integer", "int");
	typeMapping.put("UnsignedInteger", "uint");
	typeMapping.put("UnsignedLong", "ulong");
	typeMapping.put("long", "long");
	typeMapping.put("float", "float");
	typeMapping.put("double", "double");
	typeMapping.put("number", "decimal");
	typeMapping.put("decimal", "decimal");
	typeMapping.put("BigDecimal", "decimal");
	typeMapping.put("DateTime", this.useDateTimeOffsetFlag ? "DateTimeOffset" : "DateTime");
	typeMapping.put("date", this.useDateOnly() ? "DateOnly" : "DateTime");
	typeMapping.put("file", "System.IO.Stream");
	typeMapping.put("array", "List");
	typeMapping.put("list", "List");
	typeMapping.put("map", "Dictionary");
	typeMapping.put("object", "Object");
	typeMapping.put("UUID", "Guid");
	typeMapping.put("URI", "string");
	typeMapping.put("AnyType", "Object");
	if (this.useCollection) {
	typeMapping.put("array", "Collection");
	typeMapping.put("list", "Collection");
	}
	}

This specific code is what determines what type to pass to the generated code:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

Lines 1483 to 1491 in e87c4ea

	if (typeMapping.containsKey(openAPIType)) {
	type = typeMapping.get(openAPIType);
	String languageType = getNullableType(p, type);
	if (languageType != null) {
	return languageType;
	}
	} else {
	type = openAPIType;
	}

It first checks if the type is part of the default typemapping for C#. If not, it just assumed it's a reference type and skips the nullability check.
Even if the nullability check was accessed, the nullability check itself also just checks a list of primitive types:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

Lines 1465 to 1471 in e87c4ea

	public String getNullableType(Schema p, String type) {
	if (languageSpecificPrimitives.contains(type)) {
	return type;
	} else {
	return null;
	}
	}

Some implementations of the C# generator override the nullability check but still use a hardcoded list.
For example in the ASP.NET server generator:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AspNetServerCodegen.java

Lines 592 to 603 in e87c4ea

	@Override
	public String getNullableType(Schema p, String type) {
	if (languageSpecificPrimitives.contains(type)) {
	if (isSupportNullable() && ModelUtils.isNullable(p) && (this.getNullableTypes().contains(type) || nullReferenceTypesFlag)) {
	return type + "?";
	} else {
	return type;
	}
	} else {
	return null;
	}
	}

Since these are all hardcoded lists in all implementations, there currently is no workaround.
It's also hard to automate this since the generator has no way to determine if a custom type is a value type or reference type.

The best way to solve this without adjusting source every time seems to be exposing a (for example) additionalNullables property to the configuration to add more types to the list as needed.

[balassit]

Timespan should be represented by the OpenAPI duration property. Can this be fixed to avoid custom solutions?
https://spec.openapis.org/registry/format/#values

TTL:
  description: time to live.
  format: duration
  type: string

• @wing328

[balassit]

I created a draft PR to address my proposal [WIP][csharp] Duration format property support #20519