Accumulating sum in one line in C#

Is it possible to do this in one line with the goal of getting the accumulated sum equal to n?

int n = 0;
for (int i = 1; i <= 10; i++)
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    n += i;
}

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There's a LINQ extension method for that:

static int SlowSum1ToN(int N)
{
    return Enumerable.Range(1, N).Sum();
}

However, you can also calculate this particular value (the sum of an arithmetic sequence) without iterating at all:

static int FastSum1ToN(int N)
{
    return (N * (1 + N)) / 2;
}

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Yes it's possible, you can do that using Enumerable.Range to generate a sequence of integer numbers and then call the LINQ extension method Sum as the following:

int result = Enumerable.Range(1, 10).Sum();

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Yes

You can replace

int S = 0

for (int i = 0; i <= N; i++)
{
    S += i;
}

with

int S = (N*(N+1))/2

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As an added answer, there is also the Aggregate function which is more general than Sum:

var sum = Enumerable.Range(1, 10).Aggregate(0, (a, b) => a += b);

So you can also do stuff like

// multiply 1 * 2 * 3 * 4 ...
var product = Enumerable.Range(1, 10).Aggregate(1, (a, b) => a *= b);

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What's the fascination with one liners? Here it is:

int n = 0; for (int i = 1; i <= 10; i++) {n += i;}

Is there a better way to calculate this? Sure. See the other answers that use a formula.