C# Cheat Sheet
by Sam AllenProblem. Review syntax and method names in the C# programming language. View arrays, strings, methods, sorting, and classes. Provide brief examples of each task, useful for homework or reference. Solution. This table contains many common tasks in C#, with columns containing the title, description, and example code.
| Arrays | ||
| Create new array | Create new array to store values. | int[] cat = new int[5]; // cat = 0, 0, 0, 0, 0 string[] s = new string[10]; // s = null, null, ... |
| Initialize array | Create new int or string array with certain values. | int[] cat = {1, 4, 6}; string[] a = {"dog", "cat", "plant"}; |
| Assign array | Set element in array to value. | cat[0] = 3; cat[1] = 4; cat[4] = 7; |
| Check array size | Use array's length property, then access elements. | if (cat.Length == 3) { // 3 elements } |
| Sort array | Sort an array alphabetically (A - Z). | string[] a = new string[] { "z", "a", "b" }; Array.Sort(a); // "a", "b", "z" |
| Use 2D array | Use a two-dimensional array to store a grid of values. | int[,] i = new int[2, 2]; i[0, 0] = 0; i[0, 1] = 1; i[1, 0] = 2; i[1, 1] = 3; // 0, 1 // 2, 3 |
| Convert List to array | Use extension method ToArray() to convert List to equivalent array. | List<int> e = new List<int>(); e.Add(1); e.Add(2); int[] a = e.ToArray(); // a = 1, 2 |
| Loop through List | Iterate through each item in an array or list. | List<int> e = new List<int>(); e.Add(1); e.Add(2); foreach (int i in e) { // 1, 2 } |
| Reverse array | Reorder elements in array backwards. | int[] a = {5, 6, 1}; Array.Reverse(a); // 1, 6, 5 |
| Methods | ||
| Ref parameter | Allow another method to directly change value. | class C { void Method() { int a = 4; Method2(ref a); // a = 5 } void Method2(ref int p) { p = 5; } } |
| Out parameter | Allow another method to directly change value. With compile-time checking. | class C { void Method() { int a; Method2(out a); // a = 5 } void Method2(out int p) { p = 5; } } |
| Strings | ||
| Split string | Divide string into separate parts. | string c = "one,two"; string[] s = c.Split(','); // s[0] = "one" // s[1] = "two" |
| String new lines | Declare a string with newlines in it. Use "" for a quote. | string s = @"line 1 line 2 line 3"; |
| Combine strings | Add strings together (also called concatenation). Use overloaded + operator. | string s1 = "cat"; string s2 = "dog"; string c = s1 + " and " + s2; // c = "cat and dog" |
| Compare strings | See if two strings have equal characters and lengths. | string s1 = "cat"; string s2 = "dog"; string s3 = "cat"; if (s1 == s2) { // not true } if (s1 == s3) { // success } |
| Empty string check | See if string is empty or null (has no value). | string s1 = ""; string s2 = null; string s3 = "cat"; if (string.IsNullOrEmpty(s1)) { // true } if (string.IsNullOrEmpty(s2)) { // true } if (string.IsNullOrEmpty(s3)) { // not true } |
| Get string length | Find number of characters in string. | string c = "cat"; if (c.Length == 3) { // true } |
| Append strings quickly | Use StringBuilder and convert back to string. | StringBuilder b = new StringBuilder(); b.Append("Text"); b.Append(" more"); string r = b.ToString(); // r = "Text more" |
| Uppercase entire string | Uppercase each letter in string. | string s = "cat"; s = s.ToUpper(); // s = "CAT" |
| Uppercase first letter | Uppercase the first letter in string. | string s = "cat"; char[] a = s.ToCharArray(); a[0] = char.ToUpper(a[0]); string u = new string(a); // u = "Cat" |
| Convert string to int | Take string containing digits and convert it to int value. | string s = "105"; int i = int.Parse(s); // i = 105 string s2 = "105.5"; double d = double.Parse(s2); // d = 105.5 |
| Substring of string | Get part of string based on indexes. | string s = "developer"; string b = s.Substring(0, 7); // b = "develop" |
| Remove whitespace | Trim whitespace at beginning and ending of string. | string s = " cat "; string t = s.Trim(); // t = "cat" |
| Change string characters | Modify the letters in string in-place. | string s = "cat"; char[] a = s.ToCharArray(); // a = 'c', 'a', 't' a[0] = 'h'; // a = 'h', 'a', 't' string s2 = new string(a); // s2 = "hat" |
| Letter position | Find position of letter in string using IndexOf. | string s = "word"; int i = s.IndexOf("r"); // i = 2 |
| Classes | ||
| Declare constructor | Create new constructor for class. | class C { public C(int a) { // initialize } } |
| Cast safely | Use as operator or is operator. Convert from one type to another. | void Method(object a) { if (a is MyClass) { // correct type } } void Method2(object a) { MyClass m = a as MyClass; if (m != null) { // correct type } } |
| Null reference | Causes exception in programs. Null variable used. | string s = null; if (s.Length == 0) { // exception raised } |
| New object | Create a new object of a kind. | class C { // impl. } class Program { void Main() { C name = new C(); } } |
| Singleton | Make a single object. One instance per AppDomain. | class S { private static readonly _inst = new S(); public static S Instance { get { return _inst; } } S() { // init } } |
| Properties Accessors Getters, setters | Create properties to access fields of classes publicly. | class C { public int P { get; set; } } void Method(C name) { name.P = 4; int i = name.P; // i = 4 } |
| File IO | ||
| Read file lines | Read in each line of file into array. | string[] lines = File.ReadAllLines("file.txt"); // lines[0] = "..." // lines[1] = "..." |
| Read text file | Read in entire file containing text. | string f = File.ReadAllText("file.txt"); // f = "..." |
| Read lines separately | Read file line-by-line with StreamReader. Fastest and lowest memory. | using (StreamReader s = new StreamReader("file.txt")) { string t; while ((t = s.ReadLine()) != null) { // t = "..." } } |
| Append to file | Add text to end of file on disk. | File.AppendAllText("file.txt", "..." + Environment.NewLine); |
| Write file | Write text to file on disk, replacing any existing files. | File.WriteAllText("file.txt", "..."); |
| Directory exists | See if folder exists on file system. Add using System.IO; | if (Directory.Exists("C:\\f")) { // ... } |
| File exists | See if file exists on disk at path. | if (File.Exists("file.txt")) { // ... } |
| Hashtables | ||
| Use hashtable Use Dictionary | Use the generic Dictionary object with string keys. | Dictionary<string, int> d = new Dictionary<string, int>(); d.Add("cat", 2); d.Add("dog", 4); if (d.ContainsKey("cat")) { // true } if (d.ContainsKey("hat")) { // not true } |
| Lookup Dictionary value | Get value from hashtable/Dictionary based on key. | Dictionary<string, int> d = new Dictionary<string, int>(); d.Add("cat", 2); if (d.ContainsKey("cat")) { int v = d["cat"]; // v = 2 } |
| Scan Dictionary values | Use KeyValuePair on Dictionary
to list each key/value combo. Use this style to convert to string. | foreach (KeyValuePair<string, int> p in d) { // p.Key, p.Value } |
| Language | ||
| Namespaces | Put at top of file. Required for some examples. | using System; using System.IO; using System.Linq; using System.Text; |
| Current time | Get current time using DateTime. | DateTime d = DateTime.Now; string s = d.ToString(); // s = "8/20/2008 4:18:52 PM" |
| Debug message | Write diagnostics message to console. | using System.Diagnostics; class C { void Method() { Debug.WriteLine("..."); } } |
| Use constants Enum values | Use enum type to assign names to numbers. | enum E { None, Cat, Dog }; void Method() { E name = E.Cat; if (name == E.Dog) { // not true } } |
| Loop through numbers | Use the for loop to loop through range of values. | for (int i = 0; i < 100; i++) { // 0, 1, 2, 3, ... 99 } |
| Catch exceptions | Detect errors and try to recover from them. | try { int i = 1 / 0; } catch (Exception) { // log error } |
| Switch statement | Compare value against constant values. Faster than if. | switch(v) { case "cat": case "tiger": { // feline break; } case "dog": default: { // other break; } } |
| Interfaces | ||
| Interfaces Code contracts | Define required methods for classes so they can be swapped. | public interface IName { void Method(); } class C : IName { public void Method() { // ... } } class D : IName { public void Method() { // ... } } |
| Use interfaces | Use classes by their common interfaces. Improves code reuse. | void Method() { C name = new C(); Method2((IName)name); D name2 = new D(); Method2((IName)name2); } void Method2(IName i) { // ... } |
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