Code Snippets Tutorial Part 1

Lambda expression common syntax-CSharp

There are multiple ways of expressing lambdas, depending on the exact scenario - some examples:

// simplest form; no types, no brackets
Func f1 = x => 2 * x;
// optional exlicit argument brackets
Func f2 = (x) => 2 * x;
// optional type specification when used with brackets
Func f3 = (int x) => 2 * x;
// multiple arguments require brackets (types optional)
Func f4 = (x, y) => x * y;
// multiple argument with explicit types
Func f5 = (int x, int y) => x * y;

The signature of the lambda must match the signature of the delegate used (whether it is explicit, like above, or implied by the context in things like .Select(cust => cust.Name)

You can use lambdas without arguments by using an empty expression list:

// no arguments
Func f0 = () => 12;

Ideally, the expression on the right hand side is exactly that; a single expression. The compiler can convert this to either a delegate or an Expression tree:

// expression tree
Expression> f6 = (x, y) => x * y;

However; you can also use statement blocks, but this is then only usable as a delegate:

// braces for a statement body
Func f7 = (x, y) => {
int z = x * y;
Console.WriteLine(z);
return z;
};

Note that even though the .NET 4.0 Expression trees support statement bodies, the C# 4.0 compiler doesn't do this for you, so you are still limited to simple Expression trees unless you do it "the hard way"; see my article on InfoQ for more information.

Calculate relative time

const int SECOND = 1;
const int MINUTE = 60 * SECOND;
const int HOUR = 60 * MINUTE;
const int DAY = 24 * HOUR;
const int MONTH = 30 * DAY;

if (delta < 0)
{
  return "not yet";
}
if (delta < 1 * MINUTE)
{
  return ts.Seconds == 1 ? "one second ago" : ts.Seconds + " seconds ago";
}
if (delta < 2 * MINUTE)
{
  return "a minute ago";
}
if (delta < 45 * MINUTE)
{
  return ts.Minutes + " minutes ago";
}
if (delta < 90 * MINUTE)
{
  return "an hour ago";
}
if (delta < 24 * HOUR)
{
  return ts.Hours + " hours ago";
}
if (delta < 48 * HOUR)
{
  return "yesterday";
}
if (delta < 30 * DAY)
{
  return ts.Days + " days ago";
}
if (delta < 12 * MONTH)
{
  int months = Convert.ToInt32(Math.Floor((double)ts.Days / 30));
  return months <= 1 ? "one month ago" : months + " months ago";
}
else
{
  int years = Convert.ToInt32(Math.Floor((double)ts.Days / 365));
  return years <= 1 ? "one year ago" : years + " years ago";
}

* 2 hours ago
* 3 days ago
* a month ago