ToLower, ToUpper

Every letter has an uppercase and lowercase form. With the C# ToLower and ToUpper methods on the string type, we can convert cases.

Method notes

ToLower changes strings to be all lowercase. It converts an entire string—without changing letters that are already lowercased or digits.

ToLower example

We call ToLower on a string—it will return a copied version that is all lowercase. Characters such as digits will not be modified. Only uppercase letters are changed.

using System;

// Input string.
string mixedCase = "CATx2";

// Call ToLower method.
// ... This returns a new string copy.
string lower = mixedCase.ToLower();

// Display results.
Console.WriteLine("BEFORE: " + mixedCase);
Console.WriteLine(" AFTER: " + lower);
BEFORE: CATx2
 AFTER: catx2

Example, CultureInfo

Next we use CultureInfo with ToLower. This example has improved performance over the code in the first example. This is shown in the benchmark later.

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        // Input string.
        string upper = "UPPERCASE STRING";

        // Get current culture.
        CultureInfo culture = CultureInfo.CurrentCulture;

        // Call ToLower instance method with globalization parameter.
        string lower = upper.ToLower(culture);

        // Display result.
        Console.WriteLine(lower);
    }
}
uppercase string

ToUpper example

ToUpper uppercases all letters in a string. It is useful for processing text input or for when you need to check the string against an already uppercase string.

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        //
        // Uppercase this mixed case string.
        //
        string value1 = "Lowercase string.";
        string upper1 = value1.ToUpper();
        Console.WriteLine(upper1);

        //
        // Uppercase this string.
        //
        string value2 = "ABC123";
        string upper2 = value2.ToUpper(CultureInfo.InvariantCulture);
        Console.WriteLine(upper2);
    }
}
LOWERCASE STRING.
ABC123

ToLowerInvariant

ToLowerInvariant and ToUpperInvariant affect strings differently than ToLower and ToUpper. The word "invariant" indicates the system's culture has no effect on the result.

using System;

class Program
{
    static void Main()
    {
        // This demonstrates the invariant methods.
        // ... They act in the expected way.
        string test1 = "Cat";
        Console.WriteLine(test1.ToLowerInvariant());
        Console.WriteLine(test1.ToUpperInvariant());
    }
}
cat
CAT

Benchmark

Here I tested the performance of ToLower against the performance of ToLowerInvariant. I found a difference between the 2 methods.

using System;
using System.Diagnostics;
using System.Globalization;

string text = "This is an UPPER string.";
CultureInfo c = CultureInfo.CurrentCulture;

const int m = 1000000;
// Version 1: ToLower with no arguments.
Stopwatch s1 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    string text2 = text.ToLower();
    if (text2 == null)
    {
        return;
    }

}
s1.Stop();
// Version 2: ToLowerInvariant.
Stopwatch s2 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    string text2 = text.ToLowerInvariant();
    if (text2 == null)
    {
        return;
    }
}
s2.Stop();
// Version 3: ToLower with CultureInfo argument.
Stopwatch s3 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    string text2 = text.ToLower(c);
    if (text2 == null)
    {
        return;
    }
}
s3.Stop();
Console.WriteLine(s1.ElapsedMilliseconds);
Console.WriteLine(s2.ElapsedMilliseconds);
Console.WriteLine(s3.ElapsedMilliseconds);
55 ms    ToLower
35 ms    ToLowerInvariant
34 ms    ToLower(CultureInfo)

IsLower, IsUpper

If a string is already lowercase, we can simply do nothing. We must first scan for validity. This optimization can help if our data is usually already lowercase.

A summary

We used ToLower and ToUpper to transform the casing of the letters in C# strings. We also used and timed ToLowerInvariant.