Nullable datatype

Nullable types are instances of the System.Nullable struct. A nullable type can represent the normal range of values for its underlying value type, plus an additionalnull value. For example, a Nullable, pronounced "Nullable of Int32," can be assigned any value from -2147483648 to 2147483647, or it can be assigned thenull value. A Nullable can be assigned the values true or false, or null. The ability to assign null to numeric and Boolean types is particularly useful when dealing with databases and other data types containing elements that may not be assigned a value. For example, a Boolean field in a database can store the valuestrue or false, or it may be undefined.


Note: int? is the short syntax for Nullable.

Nullable is a structure that allows a value type to handle a null value.

class NullableExample
{
    static void Main()
    {
        int? num = null;
        if (num.HasValue == true)
        {
            System.Console.WriteLine("num = " + num.Value);
        }
        else
        {
            System.Console.WriteLine("num = Null");
        }

        //y is set to zero
        int y = num.GetValueOrDefault();

        // num.Value throws an InvalidOperationException if num.HasValue is false
        try
        {
            y = num.Value;
        }
        catch (System.InvalidOperationException e)
        {
            System.Console.WriteLine(e.Message);
        }
    }
}

const vs. readonly

const vs. readonly

const and readonly perform a similar function on data members, but they have a few important differences.

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const

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the constkeyword and must be initialized as they are declared. For example;

public class MyClass
{
  public const double PI = 3.14159;
}

PI cannot be changed in the application anywhere else in the code as this will cause a compiler error.

Constants must be a value type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, orbool), an enumeration, a string literal, or a reference to null.

Since classes or structures are initialized at run time with the new keyword, and not at compile time, you can't set a constant to a class or structure.

Constants can be marked as public, private, protected, internal, or protected internal.

Constants are accessed as if they were static fields, although they cannot use the static keyword.

To use a constant outside of the class that it is declared in, you must fully qualify it using the class name.

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readonly

A read only member is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor as well being able to be initialized as they are declared. For example:

public class MyClass
{
  public readonly double PI = 3.14159;
}

or

public class MyClass
{
  public readonly double PI;
 
  public MyClass()
  {
    PI = 3.14159;
  }
}

Because a readonly field can be initialized either at the declaration or in a constructor, readonly fields can have different values depending on the constructor used. A readonly field can also be used for runtime constants as in the following example:

public static readonly uint l1 = (uint)DateTime.Now.Ticks;

Notes

• readonly members are not implicitly static, and therefore the static keyword can be applied to a readonly field explicitly if required.

• A readonly member can hold a complex object by using the new keyword at initialization.

• readonly members cannot hold enumerations.

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static

Use of the static modifier to declare a static member, means that the member is no longer tied to a specific object. This means that the member can be accessed without creating an instance of the class. Only one copy of static fields and events exists, and static methods and properties can only accessstatic fields and static events. For example:

public class Car
{
  public static int NumberOfWheels = 4;
}

The static modifier can be used with classes, fields, methods, properties, operators, events and constructors, but cannot be used with indexers, destructors, or types other than classes.

static members are initialized before the static member is accessed for the first time, and before the static constructor, if any is called. To access a staticclass member, use the name of the class instead of a variable name to specify the location of the member. For example:

int i = Car.NumberOfWheels;

Index - SQL Server

Indexes are created on columns in tables or views. The index provides a fast way to look up data based on the values within those columns. For example, if you create an index on the primary key and then search for a row of data based on one of the primary key values, SQL Server first finds that value in the index, and then uses the index to quickly locate the entire row of data. Without the index, a table scan would have to be performed in order to locate the row, which can have a significant effect on performance.
You can create indexes on most columns in a table or a view

Clustered Indexes

       A clustered index stores the actual data rows at the leaf level of the index. 

An important characteristic of the clustered index is that the indexed values are sorted in either ascending or descending order. As a result, there can be only one clustered index on a table or view. In addition, data in a table is sorted only if a clustered index has been defined on a table.

Note: A table that has a clustered index is referred to as a clustered table. A table that has no clustered index is referred to as a heap

Nonclustered Indexes

Unlike a clustered indexed, the leaf nodes of a nonclustered index contain only the values from the indexed columns and row locators that point to the actual data rows, rather than contain the data rows themselves. This means that the query engine must take an additional step in order to locate the actual data.

Nonclustered indexes cannot be sorted like clustered indexes; however, you can create more than one nonclustered index per table or view. SQL Server 2005 supports up to 249 nonclustered indexes, and SQL Server 2008 support up to 999. This certainly doesn’t mean you should create that many indexes. Indexes can both help and hinder performance.

Index Types

In addition to an index being clustered or nonclustered, it can be configured in other ways:

• Composite index: An index that contains more than one column. In both SQL Server 2005 and 2008, you can include up to 16 columns in an index, as long as the index doesn’t exceed the 900-byte limit. Both clustered and nonclustered indexes can be composite indexes.
• Unique Index: An index that ensures the uniqueness of each value in the indexed column. If the index is a composite, the uniqueness is enforced across the columns as a whole, not on the individual columns. For example, if you were to create an index on the FirstName and LastName columns in a table, the names together must be unique, but the individual names can be duplicated.

A unique index is automatically created when you define a primary key or unique constraint:

• Primary key: When you define a primary key constraint on one or more columns, SQL Server automatically creates a unique, clustered index if a clustered index does not already exist on the table or view. However, you can override the default behavior and define a unique, nonclustered index on the primary key.
• Unique: When you define a unique constraint, SQL Server automatically creates a unique, nonclustered index. You can specify that a unique clustered index be created if a clustered index does not already exist on the table.

• Covering index: A type of index that includes all the columns that are needed to process a particular query. For example, your query might retrieve the FirstName and LastName columns from a table, based on a value in the ContactID column. You can create a covering index that includes all three columns.