[BUG][C#] Wrong type in default constructor for nullable enumerations

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When an entity has a nullable enumeration member, the C# code generator creates a default constructor that requires a non-nullable value for the member. This produces an error during deserialization when the serialized entity has a null value: Cannot convert null value to <enumeration type> because the constructor requires a non-null value.

openapi-generator version

openapi-generator-cli-4.2.3

OpenAPI declaration file content or url

https://gist.github.com/simonhaines/3cf34ff70d5bf75e7d3d7009bc6979b6

Command line used for generation

java -jar openapi-generator-cli-4.2.3.jar generate -i spec.json -g csharp-netcore

Steps to reproduce

1. Download the spec file from the gist
2. Use the above command line to generate models

Actual output (extract from CoffeeEntity.cs)

/// <summary>
/// Gets or Sets Bean
/// </summary>
[DataMember(Name="bean", EmitDefaultValue=true)]
public CoffeeBean? Bean { get; set; }
/// <summary>
/// Initializes a new instance of the <see cref="CoffeeEntity" /> class.
/// </summary>
/// <param name="bean">bean.</param>
public CoffeeEntity(CoffeeBean bean = default(CoffeeBean))
{
    this.Bean = bean;
}

Expected output (note nullable type in constructor)

/// <summary>
/// Gets or Sets Bean
/// </summary>
[DataMember(Name="bean", EmitDefaultValue=true)]
public CoffeeBean? Bean { get; set; }
/// <summary>
/// Initializes a new instance of the <see cref="CoffeeEntity" /> class.
/// </summary>
/// <param name="bean">bean.</param>
public CoffeeEntity(CoffeeBean? bean = default(CoffeeBean?))
{
    this.Bean = bean;
}

Related issues/PRs

Maybe https://github.com/OpenAPITools/openapi-generator/issues/4816

Suggest a fix

It looks like the expected output can be achieved by setting the required property of the member metadata to false so that this line of the template appends the ? to the type name.

Start with modules/openapi-generator/src/main/resources/csharp/modelGeneric.mustache, especially the constructor template line linked in the issue. Generate the C# models from the provided gist with openapi-generator-cli-4.2.3 and inspect CoffeeEntity.cs. Done means a nullable enumeration member produces a nullable constructor parameter and deserialization accepts a null value.