c#–组合两个linq表达式以注入导航属性

我有相同的条件集直接应用于一个类或具有相同导航属性的其他类.

例如,我有家和我有代理,代理与家庭有联系.

所以我正在寻找一个代理商名称为“a”的房屋,或者我正在寻找名为“a”的代理商,查询如下,

Expression<> ax = x=> x.Name == "a" ;
Expression> hx = x=> x.Agent.Name == "a";


我有近100个Agent的搜索查询,我也必须将它们应用于Home queryable.我不介意再写一遍,但它很难维护,因为我们知道它们会在开发过程中经常变化.

我想要做的是,我想为这样的Home查询编写表达式,

Expression> hx = Combine( x=>x.Agent , x=>x.Name == "a");


Combine将在哪里,

Expression> Combine(
Expression> parent,
Expression> nav){
// combine above to form...
(x=>x.Agent , x=>x.Name == "a")
=> x => x.Agent.Name == "a"
(x=>x.Agent, x=>x.Name.StartsWith("a") || x.Name.EndsWith("a"))
=> x=>x.Agent.Name.StartsWith("a") || x.Agent.Name.EndsWith("a")
// look carefully, x gets replaced by x.Agent in every node..
// I know very little about expression rewriting, so I need little help
}


解决方法:

是的,您确实需要访问者来替换部分原始表达式.你可以这样做：

Expression> Combine(Expression> parent, Expression> nav)
{
var param = Expression.Parameter(typeof(T), "x");
var visitor = new ReplacementVisitor(parent.Parameters[0], param);
var newParentBody = visitor.Visit(parent.Body);
visitor = new ReplacementVisitor(nav.Parameters[0], newParentBody);
var body = visitor.Visit(nav.Body);
return Expression.Lambda>(body, param);
}
public class ReplacementVisitor : System.Linq.Expressions.ExpressionVisitor
{
private readonly Expression _oldExpr;
private readonly Expression _newExpr;
public ReplacementVisitor(Expression oldExpr, Expression newExpr)
{
_oldExpr = oldExpr;
_newExpr = newExpr;
}
public override Expression Visit(Expression node)
{
if (node == _oldExpr)
return _newExpr;
return base.Visit(node);
}
}


用法示例：

Expression> expr1 = f => f.Bar;
Expression> expr2 = b => b.Baz;
var expr = Combine(expr1, expr2); // f => f.Bar.Baz