C# Count Vowels

Right now, you’re checking whether the sentence as a whole contains any vowels, once for each character. You need to instead check the individual characters.

   for (int i = 0; i < sentence.Length; i++)
    {
        if (sentence[i]  == 'a' || sentence[i] == 'e' || sentence[i] == 'i' || sentence[i] == 'o' || sentence[i] == 'u')
        {
            total++;
        }
    }

That being said, you can simplify this quite a bit:

static void Main()
{
    int total = 0;
    // Build a list of vowels up front:
    var vowels = new HashSet<char> { 'a', 'e', 'i', 'o', 'u' };

    Console.WriteLine("Enter a Sentence");
    string sentence = Console.ReadLine().ToLower();

    for (int i = 0; i < sentence.Length; i++)
    {
        if (vowels.Contains(sentence[i]))
        {
            total++;
        }
    }
    Console.WriteLine("Your total number of vowels is: {0}", total);

    Console.ReadLine();
}

You can simplify it further if you want to use LINQ:

static void Main()
{
    // Build a list of vowels up front:
    var vowels = new HashSet<char> { 'a', 'e', 'i', 'o', 'u' };

    Console.WriteLine("Enter a Sentence");
    string sentence = Console.ReadLine().ToLower();

    int total = sentence.Count(c => vowels.Contains(c));
    Console.WriteLine("Your total number of vowels is: {0}", total);
    Console.ReadLine();
}