忽略where子句中的空参数linq to sql

Question

我需要忽略 where 子句中的空参数，以便我可以从用户端使用应用的过滤器获取适当的数据。 为了实现这一点，我目前正在使用 if..else 嵌套方法，该方法随着参数数量的增加而增加。 我想知道是否有其他有效的方法可以通过避免行数和复杂性并提高可读性来处理这种情况

public List<Members> GetMembers(int currentPosition, string memberStatus,
                string package, string packageStatus, string branch)
{
    var members = new List<Members>();
    if (package != null)
    {
        //include package
        if (packageStatus != null)
        {
            //include package, packageStatus
            if (branch != null)
            {
                //include package,packageStatus,branch
                members = db.Members.Where(x => x.PackageName == package && x.PackageStatus == packageStatus && x.Branch == branch).ToList();
            }
            else
            {
                //include package,packageStatus
                members = db.Members.Where(x => x.PackageName == package && x.PackageStatus == packageStatus).ToList();
            }
        }
        else
        {
            if (branch != null)
            {
                //include package,branch
                members = db.Members.Where(x => x.PackageName == package && x.Branch == branch).ToList();
            }
            else
            {
                //include package
                members = db.Members.Where(x => x.PackageName == package).ToList();
            }
        }
    }
    else
    {
        if (packageStatus != null)
        {
            //include packageStatus
            if (branch != null)
            {
                //include packageStatus,branch
                members = db.Members.Where(x => x.PackageStatus == packageStatus && x.Branch == branch).ToList();
            }
            else
            {
                //include packageStatus
                members = db.Members.Where(x => x.PackageStatus == packageStatus).ToList();
            }
        }
        else
        {
            if (branch != null)
            {
                //include packageStatus,branch
                members = db.Members.Where(x => x.PackageStatus == packageStatus && x.Branch == branch).ToList();
            }
            else
            {
                //include nothing
                members = db.Members.ToList();
            }
        }
    }

    return members;
}

Answer 1

您可以将这些条件添加到查询中。 它不会成为最具可读性的 SQL，但假设您发现可读代码更重要并且信任 SQL Server 的优化器：

members = db.Members.Where(x =>
    (package == null || x.PackageName == package) &&
    (packageStatus == null || x.PackageStatus == packageStatus) &&
    (branch == null || x.Branch == branch)
).ToList();
        

或者，您可以有条件地将Where() s 附加到IQueryable<Member>类型的变量。 参见示例实体框架：条件过滤器。

Answer 2

我想建议该方法的可读版本：

public List<Members> GetMembers(int currentPosition, string memberStatus,
    string package, string packageStatus, string branch)
{
    var members = new List<Members>();

    members = db.Members.ToList();

    if (package != null)
    {
        members = members.Where(x => x.PackageName == package);
    }

    if (packageStatus != null)
    {
        members = members.Where(x => x.PackageStatus == packageStatus);
    }

    if (branch != null)
    {
        members = members.Where(x => x.Branch == branch);
    }

    return members.ToList();
}

Answer 3

我认为这有效（还没有尝试编译它，但在我的脑海中它是有道理的）

    public List<Members> GetMembers(int currentPosition, string memberStatus,
                string package, string packageStatus, string branch)
    {
        List<Members> ret = db.Members()
        if(memberStatus != null || package != null || packageStatus != null || branch != null)
        {
            ret = db.Members.Where(x => 
                (memberStatus == null) ? (true) : (memberStatus == x.MemberStatus)
                && (package == null) ? (true) : (package == x.PackageName)
                && (packageStatus == null) ? (true) : (packageStatus == x.PackageStatus)
                && (branch == null) ? (true) : (branch == x.Branch)
            ).ToList();
        }
        return ret;
    }