var rightExp = Expression.Invoke(rightFunc, pExp);
// (a => leftFunc(a))(x) && (a => rightFunc(a))(x)
var bodyExp = Expression.AndAlso(leftExp, rightExp);
// x => (a => leftFunc(a))(x) && (a => rightFunc(a))(x)
var resultEx
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p = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
return resultExp;
}
}
只听枯叉一声黑暗渐渐地就分开
但是，上面的方法，其实可以在优化一下。避免对左右表达式的直接调用。
使用一个叫做 Unwrap 的方法，可以将 Lambda Expression 解构成只包含 Body 部分的表达式。
这是一个自定义的扩展方法，你可以通过 ObjectVisitor[1] 来引入这个方法。
限于篇幅，我们此处不能展开谈 Unwrap 的实现。我们只需要关注和前一个示例中注释的不同即可。
[Test]
public void Expression05()
{
var filter = CreateFilter(x => x >= 1, x => x < 5);
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Expression<Func<int, bool>> CreateFilter(Expression<Func<int, bool>> leftFunc,
Expression<Func<int, bool>> rightFunc)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
// leftFunc(x)
var leftExp = leftFunc.Unwrap(pExp);
// rightFunc(x)
var rightExp = rightFunc.Unwrap(pExp);
// leftFunc(x) && rightFunc(x)
var bodyExp = Expression.AndAlso(leftExp, rightExp);
// x => leftFunc(x) && rightFunc(x)
var resultExp = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
return resultExp;
}
}
天和地分开后盘古怕它们还合并
我们可以再优化以下，把 CreateFilter 方法扩展为支持多个子表达式和可自定义子表达式的连接方式。
于是，我们就可以得到一个 JoinSubFilters 方法。
[Test]
public void Expression06()
{
var filter = JoinSubFilters(Expression.AndAlso, x => x >= 1, x => x < 5);
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Expression<Func<int, bool>> JoinSubFilters(Func<Expression, Expression, Expression> expJoiner,
params Expression<Func<int, bool>>[] subFilters)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
var result = subFilters[0];
foreach (var sub in subFilters[1…])
{
var leftExp = result.Unwrap(pExp);
var rightExp = sub.Unwrap(pExp);
var bodyExp = expJoiner(leftExp, rightExp);
result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
}
return result;
}
}
他就头顶着天脚蹬着地不知多久
有了前面的经验，我们知道。其实x => x >= 1这个表达式可以通过一个工厂方法来创建。
所以，我们使用一个 CreateMinValueFilter 来创建这个表达式。
[Test]
public void Expression07()
{
var filter = JoinSubFilters(Expression.AndAlso,
CreateMinValueFilter(1),
x => x < 5);
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Expression<Func<int, bool>> CreateMinValueFilter(int minValue)
{
return x => x >= minValue;
}
Expression<Func<int, bool>> JoinSubFilters(Func<Expression, Expression, Expression> expJoiner,
params Expression<Func<int, bool>>[] subFilters)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
var result = subFilters[0];
foreach (var sub in subFilters[1…])
{
var leftExp = result.Unwrap(pExp);
var rightExp = sub.Unwrap(pExp);
var bodyExp = expJoiner(leftExp, rightExp);
result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
}
return result;
}
}
盘古也累得倒下来变成山石河流
当然，可以只使用 Expression 相关的方法来创建x => x >= 1。
[Test]
public void Expression08()
{
var filter = JoinSubFilters(Expression.AndAlso,
CreateMinValueFilter(1),
x => x < 5);
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Expression<Func<int, bool>> CreateMinValueFilter(int minValue)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
// minValue
var rightExp = Expression.Constant(minValue);
// x >= minValue
var bodyExp = Expression.GreaterThanOrEqual(pExp, rightExp);
var result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
return result;
}
Expression<Func<int, bool>> JoinSubFilters(Func<Expression, Expression, Expression> expJoiner,
params Expression<Func<int, bool>>[] subFilters)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
var result = subFilters[0];
foreach (var sub in subFilters[1…])
{
var leftExp = result.Unwrap(pExp);
var rightExp = sub.Unwrap(pExp);
var bodyExp = expJoiner(leftExp, rightExp);
result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
}
return result;
}
}
那么看来盘古也吃不了上班的苦
那既然都用了 Expression 来创建子表达式了，那就干脆再做一点点改进，把x => x < 5也做成从工厂方法获取。
[Test]
public void Expression09()
{
var filter = JoinSubFilters(Expression.AndAlso,
CreateValueCompareFilter(Expression.GreaterThanOrEqual, 1),
CreateValueCompareFilter(Expression.LessThan, 5));
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Expression<Func<int, bool>> CreateValueCompareFilter(Func<Expression, Expression, Expression> comparerFunc,
int rightValue)
{
var pExp = Expression.Parameter(typeof(int), “x”);
var rightExp = Expression.Constant(rightValue);
var bodyExp = comparerFunc(pExp, rightExp);
var result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
return result;
}
Expression<Func<int, bool>> JoinSubFilters(Func<Expression, Expression, Expression> expJoiner,
params Expression<Func<int, bool>>[] subFilters)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
var result = subFilters[0];
foreach (var sub in subFilters[1…])
{
var leftExp = result.Unwrap(pExp);
var rightExp = sub.Unwrap(pExp);
var bodyExp = expJoiner(leftExp, rightExp);
result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
}
return result;
}
}
所以如果可以不做这需求就别搞
最后，我们在把子表达式的创建通过一点点小技巧。通过外部参数来决定。就基本完成了一个多 And 的值比较查询条件的动态构建。
[Test]
public void Expression10()
{
var config = new Dictionary<string, int>
{
{ “>=”, 1 },
{ “<”, 5 }
};
var subFilters = config.Select(x => CreateValueCompareFilter(MapConfig(x.Key), x.Value)).ToArray();
var filter = JoinSubFilters(Expression.AndAlso, subFilters);
var re = Enumerable.Range(0, 10).AsQueryable()
.Where(filter).ToList();
var expectation = Enumerable.Range(1, 4);
re.Should().BeEquivalentTo(expectation);
Func<Expression, Expression, Expression> MapConfig(string op)
{
return op switch
{
“>=” => Expression.GreaterThanOrEqual,
“<” => Expression.LessThan,
_ => throw new ArgumentOutOfRangeException(nameof(op))
};
}
Expression<Func<int, bool>> CreateValueCompareFilter(Func<Expression, Expression, Expression> comparerFunc,
int rightValue)
{
var pExp = Expression.Parameter(typeof(int), “x”);
var rightExp = Expression.Constant(rightValue);
var bodyExp = comparerFunc(pExp, rightExp);
var result = Expression.Lambda<Func<int, bool>>(bodyExp, pExp);
return result;
}
Expression<Func<int, bool>> JoinSubFilters(Func<Expression, Expression, Expression> expJoiner,
params Expression<Func<int, bool>>[] subFilters)
{
// x
var pExp = Expression.Parameter(typeof(int), “x”);
var result = subFilters[0];
foreach (var sub in subFilters[1…])
{
var leftExp = result.Unwrap(pExp);
var rightExp = sub.Unwrap(pExp);