Linq2Sql“成员访问类型不合法”例外

Question

我有一个带有编译查询的静态类，我想重用一些子查询。因此，我将公共部分提取到静态属性中，然后在多个查询中引用它：

public static class Query {
    // common part
    static Func<MyDataContext, string, IQueryable<UserAccount>> accounts = 
        (db, cID) => db.UserAccount
                .Where(x => x.XRef.cID == cID && bla-bla-bla);

    // one of queries that reuse 'accounts' part
    public static readonly Func<MyDataContext, string, string, bool> CheckClientIdentity =
        CompiledQuery.Compile<MyDataContext, string, string, bool>(
            (db, cID, identityName) => accounts(db, cID)
                .Any(x => x.IdentityName == identityName)
        );
}

这编译得很好，但在运行时我得到

System.InvalidOperationException：成员访问“System.String” “UserAccount”的“IdentityName”在类型上不合法 'System.Linq.IQueryable`1[用户帐户]。

在这种情况下也不例外

public static readonly Func<MyDataContext, string, string, bool> CheckClientIdentity =
    CompiledQuery.Compile<MyDataContext, string, string, bool>(
        (db, cID, identityName) => db.UserAccount
            .Where(x => x.XRef.cID == cID && bla-bla-bla)
            .Any(x => x.IdentityName == identityName)
    );

为什么？有什么解决办法吗？