Negating Func<T, bool> in lambda expression
asp.net, c#
Solution
Func<T, bool> expr = x => x.Prop != 1;
Func<T, bool> negativeExpr = value => !expr(value);
or
somelist = somelist.Where(value => !expr(value));
When using expression trees the following will do the trick:
Expression<Func<T, bool>> expr = x => x.Prop != 1;
var negativeExpr = Expression.Lambda<Func<T, bool>>(
Expression.Not(expr.Body),
expr.Parameters);
somelist = somelist.Where(negativeExpr);
To make your life easier, you can create the following extension methods:
public static Func<T, bool> Not<T>(
this Func<T, bool> predicate)
{
return value => !predicate(value);
}
public static Expression<Func<T, bool>> Not<T>(
this Expression<Func<T, bool>> expr)
{
return Expression.Lambda<Func<T, bool>>(
Expression.Not(expr.Body),
expr.Parameters);
}
Now you can do this:
somelist = somelist.Where(expr.Not());
Problem
``` Func<T, bool> expr = x => x.Prop != 1; somelist = somelist.Where(expr); ``` So far so good. But I would like to negate `expr` like this: ``` somelist = somelist.Where(!expr); ``` Which result in a compile error: `Cannot apply ! operator to operand of type Func<T, bool>`. Do I have to create another expression variable for this? ``` Func<T, bool> expr2 = x => x.Prop == 1; ```