What method in the String class returns only the first N characters?

string truncatedToNLength = new string(s.Take(n).ToArray());  

This solution has a tiny bonus in that if n is greater than s.Length, it still does the right thing.

Whenever I have to do string manipulations in C#, I miss the good old Left and Right functions from Visual Basic, which are much simpler to use than Substring.

So in most of my C# projects, I create extension methods for them:

public static class StringExtensions
{
    public static string Left(this string str, int length)
    {
        return str.Substring(0, Math.Min(length, str.Length));
    }

    public static string Right(this string str, int length)
    {
        return str.Substring(str.Length - Math.Min(length, str.Length));
    }
}

Note:
The Math.Min part is there because Substring throws an ArgumentOutOfRangeException when the input string's length is smaller than the requested length, as already mentioned in some comments under previous answers.

Usage:

string longString = "Long String";

// returns "Long";
string left1 = longString.Left(4);

// returns "Long String";
string left2 = longString.Left(100);

You can use LINQ str.Take(n) or str.SubString(0, n), where the latter will throw an ArgumentOutOfRangeException exception for n > str.Length.

Mind that the LINQ version returns a IEnumerable<char>, so you'd have to convert the IEnumerable<char> to string: new string(s.Take(n).ToArray()).

public static string TruncateLongString(this string str, int maxLength)
{
    if (string.IsNullOrEmpty(str)) return str;

    return str.Substring(0, Math.Min(str.Length, maxLength));
}

In C# 8 or later it is also possible to use a Range to make this a bit terser:

public static string TruncateLongString(this string str, int maxLength)
{
    return str?[0..Math.Min(str.Length, maxLength)];
}

Which can be further reduced using an expression body:

public static string TruncateLongString(this string str, int maxLength) =>
    str?[0..Math.Min(str.Length, maxLength)];

Note null-conditional operator (?) is there to handle the case where str is null. This replaces the need for an explict null check.