version1.cs

public Task AssignRoleToServicePrincipal(Guid objectId, string roleName, string scopePath)
{
    var tcs = new TaskCompletionSource();
    Task.Run(() =>
    {
        try
        {   
            var roleAssignmentResourceId = RoleAssignmentResource.CreateResourceIdentifier(scopePath, roleName);
            var roleAssignmentResource = _armClient.GetRoleAssignmentResource(roleAssignmentResourceId);
            var operationContent = new RoleAssignmentCreateOrUpdateContent(roleAssignmentResource.Id, objectId)
            {
                PrincipalType = RoleManagementPrincipalType.ServicePrincipal
            };
            
            var operationOutcome = roleAssignmentResource.Update(Azure.WaitUntil.Completed, operationContent);
            tcs.TrySetResult();
        }
        catch (Exception ex)
        {
            tcs.TrySetException(ex);
        }
    });

    return tcs.Task;
}