Question

My code like this:

    private void BindGvUsersInRole ()
    {
        var aRole =
            (from aspnet_Roles rol
                 in _allRoles
            where rol.RoleId.ToString() == ddlRoles.SelectedValue
            select rol).SingleOrDefault();

        this.gvUsersInRole.DataSource = aRole.aspnet_Users;
        this.gvUsersInRole.DataBind();
    }

I got an error on the last line:

Exception Details: System.InvalidOperationException: The result of a query cannot be enumerated more than once.

then I amend the function like this(thanks @VitaliyKalinin):

private void BindGvUsersInRole ()
{
    Guid  roleID=new Guid(ddlRoles.SelectedValue);

    var users =
    (
        from aspnet_Roles rol in _allRoles
        from u in rol.aspnet_Users
        where rol.RoleId == roleID
        select u
    ).ToList();

    this.gvUsersInRole.DataSource = users;
    this.gvUsersInRole.DataBind();
}

STILL SAME ERROR!!!!

what happens!?!?

the “aspnet_Users” property :

    /// <summary>
    /// 没有元数据文档可用。
    /// </summary>
    [XmlIgnoreAttribute()]
    [SoapIgnoreAttribute()]
    [DataMemberAttribute()]
    [EdmRelationshipNavigationPropertyAttribute("realtydbModel", "aspnet_UsersInRoles", "aspnet_Users")]
    public EntityCollection<aspnet_Users> aspnet_Users
    {
        get
        {
            return ((IEntityWithRelationships)this).RelationshipManager.GetRelatedCollection<aspnet_Users>("realtydbModel.aspnet_UsersInRoles", "aspnet_Users");
        }
        set
        {
            if ((value != null))
            {
                ((IEntityWithRelationships)this).RelationshipManager.InitializeRelatedCollection<aspnet_Users>("realtydbModel.aspnet_UsersInRoles", "aspnet_Users", value);
            }
        }
    }

Answer 1

You can use Single or SingleOrDefault() todo this:

// parse the string `ddlRoles.SelectedValue` into the appropriate type as `roleID`
var roleID = ParseRoleID(ddlRoles.SelectedValue);
var aRole = (from rol in aspnet_Roles.Include("aspnet_Users")
            where rol.RoleId == roleID 
            select rol).Single();

However this will throw an exception if there is more than one entity returned. If you just want the first entity that matches the case you can use First()or FirstOrDefault():

var roleID = ParseRoleID(ddlRoles.SelectedValue);
var aRole = (from rol in aspnet_Roles.Include("aspnet_Users")
            where rol.RoleId == roleID 
            select rol).First();

The OrDefaultversions of these methods make sure you don’t get an exception if there is no match. It will return null instead.

Also:
Rather than using ToString() on the RoleId parse the ddlRoles.SelectedValue into the appropriate type before you use it in the LINQ query since some methods (ToString() is one of them) are not supported within a LINQ query that executes against a DB.