将表达式参数作为属性添加到LINQ to Entities中 [英] Adding Expression argument as property in LINQ to Entities
问题描述
使用EF6,如何将给定的 Expression< Func< Row,string>>
参数绑定到现有的select表达式,而不必重写每个属性绑定使用表达式树?
Using EF6, how would I bind a given Expression<Func<Row, string>>
argument to an existing select expression, without having to rewrite every property binding using expression trees?
public IEnumerable<RowModel> GetRowModels(Expression<Func<Row, string>> textExpr)
{
return from row in MyDatabaseContext.MyTable
select new RowModel
{
RowID = row.ID,
CreatedDate = row.CreatedDate,
AnotherProperty = row.AnotherProperty,
Text = textExpr, // how do I bind this expression?
Value = row.OtherStuff.Where(os => os.ShouldUse).Select(os => os.Value).FirstOrDefault(),
AnotherValue = row.OtherStuff.Where(os => os.ShouldUseAgain).Select(os => os.Value).FirstOrDefault()
};
}
推荐答案
组合几个表达式的方法。具体来说,我们想要的是一种方法来获取映射值的表达式,然后接受接受第一个表达式的输入的表达式和第一个表达式的输出,并计算一个新值。
What you need here is a method to combine several expressions. Specifically, what we would like is a way to take an expression that maps a value and then also accept an expression that accepts the input of the first expression, and the output of the first expression, and computes a new value.
作为这种方法的一个实现,我们可以用第一个函数的主体替换第一个函数的结果的所有实例;之后,所有需要做的是确保两个表达式都使用相同的参数
实例。
As an implementation of this method we can replace all instances of "the result of the first function" with the body of the first function; after that all that needs to be done is to ensure that both expressions are using the same Parameter
instance.
public static Expression<Func<TFirstParam, TResult>>
Combine<TFirstParam, TIntermediate, TResult>(
this Expression<Func<TFirstParam, TIntermediate>> first,
Expression<Func<TFirstParam, TIntermediate, TResult>> second)
{
var param = Expression.Parameter(typeof(TFirstParam), "param");
var newFirst = first.Body.Replace(first.Parameters[0], param);
var newSecond = second.Body.Replace(second.Parameters[0], param)
.Replace(second.Parameters[1], newFirst);
return Expression.Lambda<Func<TFirstParam, TResult>>(newSecond, param);
}
以下代码用于替换另一个表达式的所有实例: / p>
The following code is used to replace all instances of an expression with another:
public static Expression Replace(this Expression expression,
Expression searchEx, Expression replaceEx)
{
return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}
internal class ReplaceVisitor : ExpressionVisitor
{
private readonly Expression from, to;
public ReplaceVisitor(Expression from, Expression to)
{
this.from = from;
this.to = to;
}
public override Expression Visit(Expression node)
{
return node == from ? to : base.Visit(node);
}
}
至于使用该函数;这很简单我们在 textExpression
上调用组合
,然后我们可以创建一个lambda,接受行和文本结果第一个表达式为参数。这可以让您编写一个几乎完全像您已经拥有的lambda,但是您可以使用text参数分配 Text
值:
As for using the function; it's simple enough. We call Combine
on your textExpression
, and then we can create a lambda accepting both the row and the text result of the first expression as parameters. This lets you write a lambda that's almost exactly like the one you already have, but where you can use the text parameter to assign the Text
value:
public IEnumerable<RowModel> GetRowModels(
Expression<Func<Row, string>> textExpr)
{
return MyDatabaseContext.MyTable.Select(
textExpr.Combine((row, text) => new RowModel
{
RowID = row.ID,
CreatedDate = row.CreatedDate,
AnotherProperty = row.AnotherProperty,
Text = text, // how do I bind this expression?
Value = row.OtherStuff.Where(os => os.ShouldUse)
.Select(os => os.Value).FirstOrDefault(),
AnotherValue = row.OtherStuff.Where(os => os.ShouldUseAgain)
.Select(os => os.Value).FirstOrDefault()
}));
}
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