C# Cheat Sheet
by Sam Allen - Updated June 13, 2009Problem. You need to review syntax and method names in the C# programming language. View arrays, strings, methods, sorting, and classes. 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). [C# Sort String Arrays - dotnetperls.com] | 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. [C# 2D Array Use - dotnetperls.com] | 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. [C# Split String Examples - dotnetperls.com] | 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. [C# StringBuilder Secrets - dotnetperls.com] | 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. [C# Uppercase First Letter - dotnetperls.com] | 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. [C# int.Parse for Integer Conversion - dotnetperls.com] | 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. [C# Trim String Tips - dotnetperls.com] | 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. [C# IndexOf String Examples - dotnetperls.com] | 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. [C# Exception Types - dotnetperls.com] | 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. [C# Singleton Class - dotnetperls.com] | 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. [Visual Studio Encapsulate Field - dotnetperls.com] | 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. [C# File Handling - dotnetperls.com] | string f =
File.ReadAllText("file.txt");
// f = "..." |
| Read lines separately | Read file line-by-line with StreamReader. Fastest and lowest memory. [C# Using StreamReader - dotnetperls.com] | 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. [C# Dictionary Examples, Keys and Values - dotnetperls.com] | 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. [C# Dictionary Lookup Overview - dotnetperls.com] | 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. [C# KeyValuePair Collection Hints - dotnetperls.com] | 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. [C# DateTime Tips and Tricks - dotnetperls.com] | DateTime d = DateTime.Now; string s = d.ToString(); // s = "8/20/2008 4:18:52 PM" |
| Debug message | Write diagnostics message to console. [C# Debug Write and Assert - dotnetperls.com] | using System.Diagnostics;
class C
{
void Method()
{
Debug.WriteLine("...");
}
} |
| Use constants Enum values | Use enum type to assign names to numbers. [C# Enum Tips and Examples - dotnetperls.com] | 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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