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();
}

Since Reed has answered your question, I will offer you another way to implement this. You can eliminate your loop by using LINQ and lambda expressions:

string sentence = "The quick brown fox jumps over the lazy dog.";
int vowelCount = sentence.Count(c => "aeiou".Contains(Char.ToLower(c)));

If you don't understand this bit of code, I'd highly recommend looking up LINQ and Lambda Expressions in C#. There are many instances that you can make your code more concise by eliminating loops in this fashion.

In essence, this code is saying "count every character in the sentence that is contained within the string "aeiou". "

That's because your if statement is always true, you need to compare the character at sentence[i], and see if it is a vowel, instead of seeing if the sentence contains a vowel.