Fail to deserialize the model when `additionalProperties` is an object

[isra-fel]

Current behavior

When a model is defined to have additionalProperties (aka it's a dictionary) and its value type is object, for example https://github.com/Azure/azure-rest-api-specs/blob/29446bf77d48b7128b0c6d587b78355c2b4dde73/specification/appconfiguration/resource-manager/Microsoft.AppConfiguration/preview/2019-11-01-preview/appconfiguration.json#L1303

those additional properties are lost during deserialization, resulting an empty dictionary.

Root cause

The generated code for deserialization is like:

internal ResourceIdentityUserAssignedIdentities(Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.Json.JsonObject json, global::System.Collections.Generic.HashSet<string> exclusions = null)
{
    bool returnNow = false;
    BeforeFromJson(json, ref returnNow);
    if (returnNow)
    {
        return;
    }
    Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.JsonSerializable.FromJson( json, ((Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.IAssociativeArray<Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Models.Api20191101Preview.IUserIdentity>)this).AdditionalProperties, null ,exclusions );
    AfterFromJson(json);
}

The third parameter of JsonSerializable.FromJson() should specify how the value object is deserialized, but it's null.

Work-around

Currently the work-around is to write a custom class to implement BeforeFromJson method. For example:

public partial class ResourceIdentityUserAssignedIdentities
{
    partial void BeforeFromJson(Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.Json.JsonObject json, ref bool returnNow)
    {
        // Perform deserialization using the appropriate object factory for user identity
        Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.JsonSerializable.FromJson(
            json,
            ((Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Runtime.IAssociativeArray<Microsoft.Azure.PowerShell.Cmdlets.AppConfiguration.Models.Api20191101Preview.IUserIdentity>)this).AdditionalProperties,
            (identity) => UserIdentity.FromJson(identity),
            null);
        returnNow = true;
    }
}