LINQ 中的条件 Where 子句

Question

假设我在网格中显示数据，并且我有许多文本框用于过滤数据。 员工 ID 的文本框。如果员工 ID 文本框为空，则不会添加 where 子句，但如果它不为空，则将为此添加 where 子句。如果薪水文本框有值或员工姓名文本框有值，我们可以过滤数据。

我尝试编写条件 LINQ 查询但出错。这是我的

var sName="";

var r = from t in TblFamilies
where 1 == 1
if(sName!="")
{
  && t.Name="Keith";
};

select new
{
    t.ID,
    t.ParentID,
    t.Name,
    t.CurDate
};

r.Dump();               

Answer 1

试试这个：-

首先选择数据：-

var r = from t in TblFamilie
select new
{
    t.ID,
    t.ParentID,
    t.Name,
    t.CurDate
};

然后就可以根据条件过滤了：-

   if (sName!="")
        r = r.Where(x => x.Name == sName);

Answer 2

如果您想将And 运算符和Or 运算符混合在一起，请在此处查看PredicateBuilder：http://www.albahari.com/nutshell/predicatebuilder.aspx

你可以简单地写成：

// begin with true if you start with And operator.
var predicate = PredicateBuilder.True<TblFamilie>();
predicate = predicate.And(t => t.CureDate < DateTime.UtcNow.AddDays(-1));
// you can mix with Or operator too.
predicate = predicate.Or(t => t.Name.Contains("blah"));

var results = context.TblFamilie
.Where(predicate)
.Select(new
{
  // your projection here...
});


// begin with false if you start with Or operator.
var predicate2 = PredicateBuilder.False<TblFamilie>();
predicate2 = predicate2.Or(t => t.CureDate < DateTime.UtcNow.AddDays(-1));
// you can mix with And operator too.
predicate2 = predicate2.And(t => t.Name.Contains("blah"));

var results = context.TblFamilie
.Where(predicate)
.Select(new
{
  // your projection here...
});


// even nesting is possible
var inner = PredicateBuilder.False<TblFamilie>();
inner = inner.Or (p => p.Name.Contains("foo"));
inner = inner.Or (p => p.Name.Contains("bar"));

var outer = PredicateBuilder.True<TblFamilie>();
outer = outer.And (p => p.CureDate > DateTime.UtcNow.AddDays(-3));
outer = outer.And (p => p.CureDate < DateTime.UtcNow.AddDays(-1));
outer = outer.And (inner);

var results = context.TblFamilie
.Where(outer)
.Select(new
{
  // your projection here...
});

更新

好的，假设您有一个Family 课程，并且您从某个地方获得了“家庭”。你可以像这样使用PredicateBuilder：

// you have 4 families from DB, API or anywhere.
var failies = new List<Family>
{
    new Family { Id = 1, ParentId = 1, Name = "foo", Birthday = new DateTime(1971, 1, 1) },
    new Family { Id = 1, ParentId = 1, Name = "bar", Birthday = new DateTime(1982, 1, 1) },
    new Family { Id = 1, ParentId = 1, Name = "foobar", Birthday = new DateTime(1993, 1, 1) },
    new Family { Id = 1, ParentId = 1, Name = "fake", Birthday = new DateTime(2000, 1, 1) },
};

// make predicate!
// if a family's Birthday is before than 1980 'or' Name contains "ke".
var predicate = PredicateBuilder.True<Family>();
predicate = predicate.And(o => o.Birthday < new DateTime(1980, 1, 1));
predicate = predicate.Or(o => o.Name.Contains("ke"));

// you should make IQueryable in order to use PredicateBuilder.
var result = failies.AsQueryable()
    .Where(predicate)
    .Select(o => new
    {
        o.Id, o.Name, o.Birthday    // only project what you want.
    })
    .ToList();

// now, result should contains "foo" and "fake".
foreach (var family in result)
{
    Debug.WriteLine("Name: " + family.Name);
}

更新2

您可以复制并粘贴到 LinqPad 以测试它的工作原理。在 LinqPad 中运行之前，

• 从以上链接下载LinqKit.dll。
• 确保按“F4”> 添加 > 浏览 > 选择 LinqKit.dll > 在“其他命名空间导入”选项卡中添加 LinqKit 命名空间。
• 在“查询”面板中，选择“C# 语句”的语言

粘贴并运行。

// you have 4 strings from DB, API or anywhere.
var strings = new List<string>
{
    "foo",
    "bar",
    "foobar",
    "fake"
};


// make predicate!
// if a string contains "oo" or "ke"
var predicate = PredicateBuilder.True<string>();
predicate = predicate.And(o => o.Contains("oo"));
predicate = predicate.Or(o => o.Contains("ke"));

// you should make IQueryable in order to use PredicateBuilder.
var result = strings.AsQueryable()
    .Where(predicate)
    .ToList();

// now, result should contains "foo", "foobar" and "fake".
foreach (var stringResult in result)
{
    Debug.WriteLine("Name: " + stringResult);
}