Adding N characters to a string
If you need to prepend a number of characters to a string you might consider String.PadLeft().

For example:

string str = "abc123";
Console.WriteLine(str);
str = str.PadLeft(10);
Console.WriteLine(str);


Will produce:

abc123
    abc123


On the efficiency of your example, StringBuilder.Append() is more efficient than StringBuilder.Insert(), so you might try something like:

public static string PrependSpaces2(string str)
{
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i &lt; (10 - str.Length); i++)
    {
        sb.Append(" ");
    }
    sb.Append(str);
    return sb.ToString();
}


EDIT As svick suggested, Append(char, repeat) is simpler and faster on the order of the native Pad methods:

public static string PrependSpaces3(string str)
    StringBuilder sb = new StringBuilder();
    if((NUM - str.Length) &gt; 0)
    {
        sb.Append(' ', (NUM - str.Length));
    }
    sb.Append(str);
    return sb.ToString();
}


Which when tested with this:

string str = "abc123";
Console.WriteLine("str: " + str);

int iterations = 100000;

string result = string.Empty;
Stopwatch s1 = Stopwatch.StartNew();
for (int i = 0; i &lt; iterations; i++)
{
    result = PrependSpaces(str);
}
s1.Stop();
Console.WriteLine(iterations + " calls to PrependSpaces took: {0:0.00 ms}", s1.Elapsed.TotalMilliseconds);

string result2 = string.Empty;
Stopwatch s2 = Stopwatch.StartNew();
for (int i = 0; i &lt; iterations; i++)
{
    result2 = PrependSpaces2(str);
}
s2.Stop();
Console.WriteLine(iterations + " calls to PrependSpaces2 took: {0:0.00 ms}", s2.Elapsed.TotalMilliseconds);

string result3 = string.Empty;
Stopwatch s3 = Stopwatch.StartNew();
for (int i = 0; i &lt; iterations; i++)
{
    result3 = str.PadLeft(NUM, ' ');
}
s3.Stop();
Console.WriteLine(iterations + " calls to String.PadLeft(" + NUM + ", ' ') took: {0:0.00 ms}", s3.Elapsed.TotalMilliseconds);

string result4 = string.Empty;
Stopwatch s4 = Stopwatch.StartNew();
for (int i = 0; i &lt; iterations; i++)
{
    result4 = PrependSpaces3(str);
}
s4.Stop();
Console.WriteLine(iterations + " calls to PrependSpaces3 took: {0:0.00 ms}", s3.Elapsed.TotalMilliseconds);


returns this (benchmarks for prepending 1000 characters):

str: abc123
100000 calls to PrependSpaces took: 20190.16 ms
100000 calls to PrependSpaces2 took: 1025.87 ms
100000 calls to String.PadLeft(1000, ' ') took: 84.32 ms
100000 calls to PrependSpaces3 took: 84.32 ms


for prepending 10 characters:

100000 calls to PrependSpaces took: 31.02 ms
100000 calls to PrependSpaces2 took: 11.41 ms
100000 calls to String.PadLeft(10, ' ') took: 4.53 ms
100000 calls to PrependSpaces3 took: 4.53 ms