1. Write a method that takes in an input `string` and returns `true` if all the characters in the `string` are unique and `false` if there is even a single repeated character.

```public bool CheckAreCharactersUnique(string word)
{
// Obviously unique
if (word == null || word.Length <= 1)
{
return true;
}

// Add all characters from word to new List, if char is already in List
// return false immediately
var chars = new List<char>();
for (int i = 0; i < word.Length; ++i)
{
var c = word[i];

if (chars.Contains(c))
{
return false;
}

}

return true;
}```

But there is easier and more realistic way to do this with LINQ:

```public bool CheckAreCharactersUnique(string word)
{
if (word == null || word.Length <= 1)
{
return true;
}

// Get all unique characters to new IEnumerable with Distinct method,
// compare with Length of word
return word.Distinct().Count() == word.Length;
}
```

2. Given an array containing 9 numbers ranging from 0 to 9, write a method to find the missing number. Assume you have 9 numbers between 0 to 9 and only one number is missing. Note that numbers might have random order.

```public int FindMissingNumber()
{
var numbers = new List<int>() { 0, 3, 5, 4, 8, 6, 7, 1, 9};

for (int i = 0; i < 10; i++)
{
if (!numbers.Contains(i))
{
return i;
}
}

// -1 means there is no missing number
return -1;
}```

The other, trickier way, is to use LINQ (or calculate sum manually with loop) to deduct sum of given numbers from sum of numbers from 0 to 9, which is 45:

```public int FindMissingNumber()
{
var numbers = new List<int>() { 0, 3, 5, 4, 8, 6, 7, 1, 9};

return 45 - numbers.Sum();
}```

3. Write a method to replace all spaces in a string with a given replacement string. Don't use built-in `Replace` method of `String` class.

```public string ReplaceWhiteSpaces(string text, string replaceWith)
{
var resultText = "";

foreach (var c in text)
{
if (c == ' ')
{
resultText += replaceWith;
}
else
{
resultText += c;
}
}

return resultText;
}```

Since String in .NET is immutable type, you might increase performance by using StringBuilder:

```public string ReplaceWhiteSpaces(string text, string replaceWith)
{
StringBuilder resultText = new StringBuilder();

foreach (var c in text)
{
if (c == ' ')
{
resultText.Append(replaceWith);
}
else
{
resultText.Append(c);
}
}

return resultText.ToString();
}
```