Question

我想为可变数量的列创建动态where子句。可以这样做吗？

我想我已经查看了与此主题相关的所有帖子，但仍无法找到答案。

var results = from table1 in dt1.AsEnumerable()
              join table2 in dt2.AsEnumerable() on table1.Field<int>("ID") equals table2.Field<int>("ID")
              where table1["ColumnA"].ToString() != table2["ColumnA"].ToString() ||
                    table1["ColumnB"].ToString() != table2["ColumnB"].ToString() || 
                    table1["ColumnC"].ToString() != table2["ColumnC"].ToString()
              select table1;

Answer 1

您可以在逻辑上添加where子句。这是一个扩展方法（这不是我的，很久以前我在StackOverflow的某个地方看过它。当我找到它时我会引用它）

Func<T, bool> And(this Func<T, bool> a, Func<T, bool> b) 
{ 
    return (T x) => a(x) && b(x); 
}

有了它，你可以迭代列并将它们添加到where子句：

Func<string, bool> whereClause = x => true;
foreach(string columnName in dt.Columns.Select(x => x.ColumnName)
{
    string first = dt.Columns[columnName].ToString();
    string second = dt2.Columns[columnName].ToString();

    whereClause = whereClause.And(x => first.Equals(second));
}

var result = (from foo in bar select fooBar).Where(whereClause);

Answer 2

如果是动态where子句，您可以使用System.Linq.Dynamic（ScottGu's blog article和nuget）：

var results = (from fruit in fruits 
    join car in cars on fruit.Id equals car.Id
    select new { fruit, car })
    .AsQueryable()
    .Where("fruit.ColA != car.ColA")
    .Where("fruit.ColB == car.ColB");

或者使用@ joe-albahari编写的extensions from PredicateBuilder动态构建表达式。例如：

var predicate = 
    PredicateBuilder
        .True<Tuple<Product, Product>>()
        .And(t => t.Item1.ColA != t.Item2.ColA)
        .And(t => t.Item1.ColB == t.Item2.ColB)
        .Compile();    

(from fruit in fruits 
    join car in cars on fruit.Id equals car.Id
    select Tuple.Create(fruit, car))
    .Where(predicate)
    .Dump();

ps：gisthub

提供完整代码

Linq C中加入System.Data.DataTable的动态where子句

2 个答案: