Syntax for calling a Lambda Expression with Delegates

Question

I found the following example which works well:

private static bool GreaterTwo(int arg)
{
    return arg > 2;
}

public static void GreaterTwoSample()
{
    Enumerable.Range(0, 10).Where(GreaterTwo);
}

What would be the syntax for using GreaterTwo with another parameter within the WHERE-Lambda:

private static bool GreaterTwoSmallerArg2(int arg, int arg2)
{
    return arg > 2 && arg < arg2;
}

UPDATE: thanks to HugoRune's answer, here is another working example with 2 parameters

public static void LimitationSample()
{
    Enumerable.Range(0, 10).Where(IsBetween(2, 5));
}
private static Func<int, bool> IsBetween(int min, int max)
{
    return x  => x > min && x < max;
}

Christos · Accepted Answer · 2014-08-21 14:15:14Z

3

You could try something like below:

public static void GreaterTwoSample(int arg2)
{
    Enumerable.Range(0, 10).Where(x=>GreaterTwoSmallerArg2(x, arg2));
}

answered Aug 21, 2014 at 14:15

Christos

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4 Comments

Selman Genç Over a year ago

+1 this is the correct answer you may want to add that Where(GreaterTwo); is a syntactic sugar for Where(x => GreaterTwo(x));

fubo Over a year ago

@Selman22 .. and i guess there is no sugar-way for a second parameter?

Athari Over a year ago

@Selman22 Behavior of these two syntaxes is similar, but different, in some cases very different.

D Stanley Over a year ago

@fubo is you mean just passing the method name instead of using a delegate, no, since the signature for GreaterTwoSmallerArg2 does not match the signature for the parameter to Where.

HugoRune · Accepted Answer · 2014-08-21 16:32:46Z

1

Enumerable.Range(0, 10).Where(...) requires a function that takes a single int as a parameter and returns a bool, there is no way around that.

The two usual ways to do this are:

pass the name of an existing function that takes an int, i.e.

Enumerable.Range(0, 10).Where(YourFunctionThatTakesAnIntAndReturnsBool);

pass a lambda expression

Enumerable.Range(0, 10).Where(x=>{Expression_that_returns_a_bool})

So the basic solution if you want to use a function that needs two integers is to pack it into a lambda expression i.e

Enumerable.Range(0, 10).Where(x=>GreaterTwoSmallerArg2(x, 5))

But if you want more syntactic sugar, you can make a function that returns a function,

private static Func<int,bool> funcGreaterTwoSmallerArg2(int arg2)
{
    return x => GreaterTwoSmallerArg2(x, arg2);
    // OR: return x => x > 2 && x < arg2;
}

Then you can use it like this:

Enumerable.Range(0, 10).Where(funcGreaterTwoSmallerArg2(5));

This syntax should be used with care, it can make the source hard to understand. But sometimes it is warranted, if you reuse a specific lambda often enough, or it is particularly complicated.

answered Aug 21, 2014 at 16:32

HugoRune

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3 Comments

fubo Over a year ago

i'm new to this, i transfered your example to one with 2 Parameters in my answer. What does x and y stand for?

HugoRune Over a year ago

The x and y stand for the two int in Func<int,int,bool>. (x,y) => x > min && x < max; is equivalent to bool myFunc(int x,int y){return x>min&&x<max;}. However the y seems unnecessary in your case, I am not sure why you would need it. And I think the example you provided will not work, Enumerable.Range(0, 10).Where() requires a Func<int,bool>, not Func<int,int,bool>

HugoRune Over a year ago

Regarding your earlier edit: There actually is an overload for Where that takes two int: the second int is the index of the element. So you would use that if for some reason you wanted to compare not only the value of each element in your list, but also its position.

Rico Suter · Accepted Answer · 2014-08-21 14:28:23Z

1

Where<T> takes a delegate that has a single T input and a bool output (aka predicate). To use a method that has two parameters you'd have to project to it using a lambda:

private static bool GreaterTwoSmallerArg2(int arg, int arg2)
{
    return arg > 2 && arg < arg2;
}

public static void GreaterTwoSample()
{
    Enumerable.Range(0, 10).Where(i => GreaterTwoSmallerArg2(i, {some other value));
}

edited Aug 21, 2014 at 14:28

Rico Suter

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answered Aug 21, 2014 at 14:25

D Stanley

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