Use the StringBuilder class for concatenating large numbers of strings

Do

var builder = new StringBuilder();
foreach (var word in words)
{
    builder.Append(word).Append(' ');
}
string result = builder.ToString();

BEST

Span<char> buffer = stackalloc char[1024];
var pos = 0;

foreach (var word in words)
{
    word.AsSpan().CopyTo(buffer.Slice(pos));
    pos += word.Length;
    buffer[pos++] = ' ';
}

string result = new string(buffer.Slice(0, pos));

Don't

string result = "";
foreach (var word in words)
{
    result += word + " ";
}