version2.cs

public Task AssignRoleToServicePrincipal(Guid objectId, string roleDefId, string scopePath)
{
    var tcs = new TaskCompletionSource();
    Task.Run(() =>
    {
        try
        {
            var scopePathResource = new ResourceIdentifier(scopePath);
            var roleDefId = $"/subscriptions/{scopePathResource.SubscriptionId}/providers/Microsoft.Authorization/roleDefinitions/{roleName}";
            var operationContent = new RoleAssignmentCreateOrUpdateContent(new ResourceIdentifier(roleDefId), objectId)
            {
                PrincipalType = RoleManagementPrincipalType.ServicePrincipal
            };

            var roleAssignmentResourceId = RoleAssignmentResource.CreateResourceIdentifier(scopePath, Guid.NewGuid().ToString());
            var roleAssignmentResource = _armClient.GetRoleAssignmentResource(roleAssignmentResourceId);
            var operationOutcome = roleAssignmentResource.Update(Azure.WaitUntil.Completed, operationContent);
            tcs.TrySetResult();
        }
        catch (Exception ex)
        {
            tcs.TrySetException(ex);
        }
    });

    return tcs.Task;
}