C# Methods Cheat Sheet

1️⃣ String Methods

Basic Operations

• s.Length

• s[i]

• s.Substring(start, length)

• s.IndexOf(charOrString)

• s.LastIndexOf(char)

• s.Contains(string)

• s.StartsWith(string)

• s.EndsWith(string)

• s.ToLower() / s.ToUpper()

• s.Trim()

Conversion

• s.ToCharArray()

• int.Parse(s)

• Convert.ToInt32(s)

• int.TryParse(s, out int result)

Split & Join

• s.Split(' ')

• string.Join(",", arr)

StringBuilder (for heavy modifications)

• Append()

• Insert(index, value)

• Remove(start, length)

• ToString()

2️⃣ Array Methods

Properties

• arr.Length

Static Array Methods

• Array.Sort(arr)

• Array.Reverse(arr)

• Array.Fill(arr, value)

• Array.Copy(source, dest, length)

• Array.IndexOf(arr, value)

3️⃣ List Methods

Creation

• new List<int>()

Add/Remove

• Add(item)

• AddRange(collection)

• Remove(item)

• RemoveAt(index)

• RemoveAll(predicate)

Access

• list[i]

• list.Count

Utility

• Sort()

• Reverse()

• Contains(value)

• IndexOf(value)

4️⃣ Dictionary<TKey, TValue>

Creation

• new Dictionary<int, int>()

Core Operations

• dict[key] = value

• dict.ContainsKey(key)

• dict.Remove(key)

• dict.TryGetValue(key, out value)

• dict.Count

5️⃣ HashSet

• Add(value)

• Remove(value)

• Contains(value)

• Count

6️⃣ Stack

• Push(value)

• Pop()

• Peek()

• Count

7️⃣ Queue

• Enqueue(value)

• Dequeue()

• Peek()

• Count

8️⃣ Math

• Math.Max(a, b)

• Math.Min(a, b)

• Math.Abs(x)

• Math.Sqrt(x)

• Math.Pow(x, y)

• Math.Ceiling(x)

• Math.Floor(x)

9️⃣ LINQ (System.Linq)

Filtering

• Where(x => condition)

Projection

• Select(x => something)

Aggregation

• Sum()

• Min()

• Max()

• Average()

• Count()

Element Retrieval

• First()

• FirstOrDefault()

• Single()

• SingleOrDefault()

• Last()

• ElementAt(index)

Ordering

• OrderBy(x => x)

• OrderByDescending(x => x)

• ThenBy()

Quantifiers

• Any()

• All(predicate)

Conversion

• ToList()

• ToArray()

• ToDictionary()

🔟 Useful Patterns

Frequency Count

• Dictionary<char, int>

• int[26] for lowercase letters

Two Pointer

• left = 0, right = n-1

Sliding Window

• HashSet<char> or Dictionary<char,int>

Prefix Sum

• int[] prefix

Sorting with Custom Comparator

• list.Sort((a, b) => a.CompareTo(b))

⚠️ Interview Advice

• Avoid heavy LINQ in interviews unless asked. Write explicit loops.

• Always mention time and space complexity.

• Prefer in-place operations when possible.

• Use TryGetValue instead of ContainsKey + index lookup for better performance.