C# 表达式树Lambda扩展(四)
<h2>一、前言</h2><p>原来盘算这篇文章在反面必要运用的时间写的,但是既然写到表达式的扩展呢,就一起写完吧。</p>
<p>看到这个标题就有一种疑问,Lambda表达式原来就是表达式树,还必要怎么扩展?那就看看下面的内容,你就知道了。</p>
<p>表达式系列目录</p>
<p>C# 表达式树讲解(一)</p>
<p>C# 表达式树遍历(二)</p>
<p>C# 表达式树分页扩展(三)</p>
<p>C# 表达式树Lambda扩展(四)</p>
<h2>二、Lambda扩展</h2>
<p>这里先不忙解答上面的题目,我们先看下这样一个应用场景。</p>
<p>一个页面的请求,内里带有一些条件查询,请求类如下</p>
public class ScoreRequest
{
public string CourseName { get; set; }
public string StudentName { get; set; }
}
<p>要求查询与课程名称和学生名称匹配的数据</p>
<p>数据源我们就以上一例子的数据源</p>
<p>数据源类</p>
<div align="center"></div><div align="center"></div>
public class ScoreClass
{
public string CourseName { get; set; }
public string StudentName { get; set; }
public decimal Score { get; set; }
}
View Code
<p>添加数据</p>
<div align="center"></div><div align="center"></div>
var datas = new List<ScoreClass>();
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生A",
Score = 60
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生B",
Score = 65
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生C",
Score = 70
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生D",
Score = 75
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生E",
Score = 80
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生F",
Score = 81
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生G",
Score = 82
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生H",
Score = 83
});
datas.Add(new ScoreClass
{
CourseName = "数学",
StudentName = "学生I",
Score = 84
});
View Code
<p>好了现在我们就查询数据</p>
var request = new ScoreRequest()
{
CourseName = "数",
StudentName = "H"
};
var resultDatas = datas.Where(e => e.CourseName.Contains(request.CourseName) && e.StudentName.Contains(request.StudentName))
.ToList();
<p>假如查询对象内里CourseName和StudentName字段都有值得话,这样写没题目。假如没值,那就最后的数据,就不正确了。</p>
<p>假如是直接拼凑sql语句,我们可以用if(String.IsNullOrEmpty())来判断,但是现在判断了,怎么拼凑Lambda表达式呢?</p>
<p>所以就必要我们对Lambda表达式进行扩展,让他支持这种情况。那上面的题目,就不用再专门答复了吧!!!!</p>
<p>创建一个LambdaExtension的类,代码如下</p>
public static class LambdaExtension
{
public static Expression<Func<T, bool>> True<T>() { return param => true; }
public static Expression<Func<T, bool>> False<T>() { return param => false; }
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.Compose(second, Expression.AndAlso);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
{
return first.Compose(second, Expression.OrElse);
}
private static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
{
var map = first.Parameters
.Select((f, i) => new { f, s = second.Parameters })
.ToDictionary(p => p.s, p => p.f);
var secondBody = PFTParameterExtension.ReplaceParameters(map, second.Body);
return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
}
private class PFTParameterExtension : ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, ParameterExpression> map;
public PFTParameterExtension()
{
}
public PFTParameterExtension(Dictionary<ParameterExpression, ParameterExpression> map)
{
this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
}
/// <summary>
/// 替换参数
/// </summary>
/// <param name="map">The map.</param>
/// <param name="exp">The exp.</param>
/// <returns>Expression</returns>
public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
{
return new PFTParameterExtension(map).Visit(exp);
}
protected override Expression VisitParameter(ParameterExpression p)
{
ParameterExpression replacement;
if (map != null && map.Count > 0 && map.TryGetValue(p, out replacement))
{
p = replacement;
}
return base.VisitParameter(p);
}
}
}
<p>这内里私有化了一个表达式树访问器,他的作用紧张是用来同步Lambda表达式内里的参数。</p>
<p>下面是调用方式</p>
var expression = LambdaExtension.True<ScoreClass>();
if (!string.IsNullOrWhiteSpace(request.CourseName))
expression = expression.And(e => e.CourseName.Contains(request.CourseName));
if (!string.IsNullOrWhiteSpace(request.StudentName))
expression = expression.And(et => et.StudentName.Contains(request.StudentName));
var resultDatas = datas.Where(expression.Compile())
.ToList();
Console.WriteLine($"查询结果:\n{string.Join("\n", resultDatas.Select(e => $"{e.StudentName} {e.CourseName} {e.Score}"))}");
<p>where条件内里只能带委托,而我们的expression是Lambda表达式,所以必要Compile进行委托编译。</p>
<p>运行结果:</p>
<p><div align="center"></div></p>
<p>细致看代码,第一个条件And内里的参数是“e”,第二个条件内里的参数是et,同一个Lambda表达式内里(这里只有一个参数),参数肯定是一致的,所以在LambdaExtension类中,在合并两个Lambda表达式的时间,就必要将参数合并成一个。</p>
<p>经过这样的扩展,我们就可以根据我们的现实情况,拼凑好必要的表达式,得到我们想要的结果。</p>
<h2>三、总结</h2>
<p>表达式树方面的讲解,终于可以告一段落了。不停后没有这样的写文章,现在以为写文章还是真的挺累的,本年中秋节的这三天,算是全部的给博客园了。不外这三天讲解的内容,基本上把反面Dapper的扩展必要用的技能都铺垫了,反面我们就继续对ORM的讲解了。其实没写一篇博文,蜗牛都会去罗列和梳理相干知识点,这也让蜗牛获益匪浅,也盼望蜗牛的博客能资助到园友,这就是所谓的“赠人玫瑰,手留余香”吧。</p><br><br/><br/><br/><br/><br/>来源:<a href="https://www.cnblogs.com/snailblog/archive/2019/09/15/11525118.html" target="_blank">https://www.cnblogs.com/snailblog/archive/2019/09/15/11525118.html</a>
页:
[1]