如何使用linq-to-sql从数据库访问记录？

Question

I am working on Linq to sql . I have created a function to search records in database. My functions is :

    public IEnumerable<Record> SearchRecord()
    {
      var records = db.Records.Where(r => r.Name == Name && r.Date == Date 
                                                && r.Country == Country);
        return records;
    }

I am trying to handle the case where if any one property out of Name, Date and Country is null. In that case i just want to remove that filter and fetch records only on the basis of non-null properties.I don`t want to add if - else conditions. How can i do this ?

Answer 1

Since LINQ uses a Fluent interface you could do something like:

public IEnumerable<Record> SearchRecord()
{
    var records = db.Records;
    if (Name != null)
    {
        records = records.Where(r => r.Name == Name);
    }
    if (Date != null)
    {
        records = records.Where(r => r.Date == Date);
    }
    if (Country != null)
    {
        records = records.Where(r => r.Country == Country);
    }
    return records;
}

I'm pretty sure this results in a less optimal SQL Query sent to the database, but it's just to illustrate the usage of fluent api in this matter.

You could also cheat with ?? , like:

public IEnumerable<Record> SearchRecord()
{
    var records = db.Records.Where(r => r.Name == (Name ?? r.Name)
                                        && r.Date == (Date ?? r.Date)
                                        && r.Country == (Country ?? r.Country));
    return records;
}

Answer 2

This should do the trick for you, basically you're going to say that if the parameter value is null OR the parameter value equals the row value then return me that row.

public IEnumerable<Record> SearchRecord()
{
    var records = db.Records.Where(r => (Name == null || r.Name == Name)
                                        && (Date == null || r.Date == Date)
                                        && (Country == null || r.Country == Country));
    return records;
}