Is your source code "pristine"?
A few minors really:


Standard C# naming convention for methods is PascalCase. So should be IsPristine.
If it's important (or nice) to know which specific sequence is yielding the false result then you should consider returning that from the method. Something like this:

public class PristineResult
{
    public readonly bool IsPristine;
    public readonly string OffendingSequence;
    public readonly string OffendingIndex;

    public PristineResult(bool isPristine, string offendingSequence = null, int offendingIndex = -1)
    {
        IsPristine = isPristine;
        OffendingSequence = offendingSequence;
        OffendingIndex = offendingIndex;
    }
}

static PristineResult IsPristine(string sourceCode)
{
    using (var csc = new CSharpCodeProvider(new Dictionary&lt;string, string&gt;() { { "CompilerVersion", "v3.5" } }))
    {
        var parameters = new CompilerParameters(new[] { "mscorlib.dll", "System.Core.dll" });
        parameters.GenerateExecutable = true;

        int length = sourceCode.Length;

        for (int i = 0; i &lt; length; i++)
        {
            for (int j = 1; j &lt; length - i; j++)
            {
                string sourceWithRemoval = sourceCode.Remove(i, j);
                CompilerResults results = csc.CompileAssemblyFromSource(parameters, sourceWithRemoval);

                if (results.Errors.Count == 0)
                {
                    return new PristineResult(false, sourceCode.Substring(i, j), i);
                }
            }
        }
    }

    return new PristineResult(true);
}