Working with Strings

A string holds text. The part that surprises people is that a string never changes once it exists.

Every method that looks like an edit — Trim, Replace, ToUpper, Substring — leaves the original alone and hands you a new string. Miss that and you will write code that runs, reports no error, and does nothing.

string reference = "inv-2041";

reference.ToUpperInvariant();              // the new string is thrown away
Console.WriteLine(reference);              // inv-2041

reference = reference.ToUpperInvariant();  // keep what came back
Console.WriteLine(reference);              // INV-2041

• ToUpperInvariant does not touch reference. It creates a second string and returns it.
• The first call is valid code. It does the work and discards the result, which is why the compiler cannot help you here.
• Replace, Trim, Substring, Insert and PadLeft all behave the same way. If the result is not assigned or passed on, nothing has happened.

using System.Text;

List<string> skus = new List<string> { "LAMP-01", "STAND-04", "TIDY-09" };

// A fixed number of pieces: interpolation reads best.
string customer = "Priya Sharma";
string city = "Pune";
string label = $"{customer}, {city}";

// Growing text in a loop: this is the version to avoid.
string slow = "";
foreach (string sku in skus)
{
    slow += sku + ",";
}

// Growing text in a loop: one buffer, appended to.
var builder = new StringBuilder();
foreach (string sku in skus)
{
    builder.Append(sku).Append(',');
}
string fast = builder.ToString().TrimEnd(',');

Console.WriteLine(label);
Console.WriteLine(fast);

• StringBuilder lives in System.Text, which is not one of the namespaces a console project includes for you, so the using directive is needed.
• The += version creates an entire new string each time round, copying everything written so far. With 10,000 SKUs that is 10,000 strings and the total copying grows with the square of the count.
• StringBuilder keeps one resizable buffer and only produces a string when you call ToString().
• Below a handful of pieces, use interpolation. It is clearer, and the compiler assembles it in a single pass rather than one string per join.

string entered = "priya.sharma@example.com";
string stored = "Priya.Sharma@Example.com";

Console.WriteLine(entered == stored);   // False

bool sameAddress = string.Equals(entered, stored, StringComparison.OrdinalIgnoreCase);
Console.WriteLine(sameAddress);         // True

Console.WriteLine(string.IsNullOrWhiteSpace("   "));                     // True
Console.WriteLine(stored.StartsWith("Priya", StringComparison.Ordinal)); // True

• For strings, == compares the characters rather than asking whether both variables point at the same object. That behaviour is specific to string; most reference types compare identity instead.
• Case-insensitive matching belongs in the comparison, not in the data. OrdinalIgnoreCase walks both strings character by character and ignores case.
• IsNullOrWhiteSpace covers null, empty and spaces-only in one call, which is what form validation usually needs.

decimal invoiceTotal = 1240.5m;
DateTime dueDate = new DateTime(2026, 3, 14);
int itemCount = 4;

Console.WriteLine($"Total: {invoiceTotal:N2}");      // Total: 1,240.50
Console.WriteLine($"Due:   {dueDate:yyyy-MM-dd}");   // Due:   2026-03-14
Console.WriteLine($"Items: {itemCount,5}");          // right-aligned across 5 characters

string exportFolder = @"C:\invoices\2026\march";  // backslashes kept as typed
Console.WriteLine(exportFolder);

• Anything after the colon inside the braces is a format string. N2 means grouped with two decimal places; yyyy-MM-dd is an explicit date layout.
• A comma before the colon sets a field width, which is how console output gets lined up into columns.
• The @ prefix stops backslashes being read as escape sequences, which makes file paths and regular expressions readable.
• Formatting with C for currency follows the machine's culture settings. For output a person reads that is usually right. For a value you store, log or compare, format with CultureInfo.InvariantCulture so it does not shift between machines.