Expression Tree

What is Expression Tree ?
Expression is not like other code, but it like binary tree data structure where each node is representing an object children(s). Expression tree is get derived from the Lambda Expression which you can see easily once you can drill down in framework class. One good reason of Expression tree is it allows to create dynamic linq queries.

Expression Class

using System.Collections.ObjectModel;

namespace System.Linq.Expressions
{
    // Summary:
    //     Represents a strongly typed lambda expression as a data structure in the
    //     form of an expression tree. This class cannot be inherited.
    //
    // Type parameters:
    //   TDelegate:
    //     The type of the delegate that the System.Linq.Expressions.Expression
    //     represents.
    public sealed class Expression : LambdaExpression
    {
        // Summary:
        //     Compiles the lambda expression described by the expression tree into executable
        //     code.
        //
        // Returns:
        //     A delegate of type TDelegate that represents the lambda expression described
        //     by the System.Linq.Expressions.Expression.
        public TDelegate Compile();
    }
}

Syntax of Expression
Expression<Func<type,returnType>> = (param) => lamdaexpresion;
NameSpace System.Linq.Expressions; contains defincation of Expression Tree class. so you need to include this namespace before using the Expression in your code.
Example

Expression<Func<int,int, int>> lambda = (a,b) => a *  b;

Expression Tree for the above expression

Install Expression Tree visualizer
If you are not having visualizer to view expression tree than you can install it by following below instruction
Download Samples : http://code.msdn.microsoft.com/csharpsamples
Compile code : Official_Visual_Studio_2008_C#_Samples\ExpressionTreeVisualizer\C#\ExpressionTreeVisualizer\
Get Dll from:
bin\Debug\expressionTreeVisualizer.dll
Install Dll on path : C:\Program Files\Microsoft Visual Studio 9.0\Common7\Packages\Debugger\Visualizers

Expression Tree Parts
Expression tree is consistence of following part, each part is shown in blow images.
Body

Parameters

NodeType and Type Of Expression

Difference Between Lamdab Expression and Expression
In above example I assigned lambda expression to Expression type, by doing that way enviroment represent lambda expression as expression tree not as lambda expression. And if you see memory represtation of expression tee its a object represention of expression which is already seen in expression tree visualizer, which is differnt than lambda epxssion IL.
when you write

Func<int,int, int> lambdaExpression = (a,b) => a *  b;

its a lambda expression.

Important point to Note here is
You cannot use lambda expressions with a statement to create an expression tree

Expression<Func<int, int, bool>> IsAGreater =
    (a, b) => { if (a > b)  return true; else return false; };

above code throw compile time error.

How to Create Expression Tree ?
Fist Way:
First way is very easy that I already sawn is above discussion "Create Lambda expression and assign it to expression."

Expression<Func<int,int, int>> lambda = (a,b) => a *  b;

Something as above.

Second Way:
To create expression you need to follow the below steps one by one. I am also going to show the functions that can be used in dynamic expression.

private void CreateExpressionTree()
 {

Create parameter for the expression.

ParameterExpression exp1 = Expression.Parameter(typeof(int), "a");
        ParameterExpression exp2 = Expression.Parameter(typeof(int), "b");

Create body of the expression, over here in this example using multiply function to get the multiplication of the two parameter.

BinaryExpression exp = Expression.Multiply(exp1,exp2);
        var lamExp = Expression.Lambda<Func<int, int, int>>                     
               (exp, new ParameterExpression[] { exp1, exp2 });

Compile method compile the expression tree and Invoke allows to execute the compiled expression tree. As you see you need to pass the values for the parameter of expression tree.

int c = (lamExp.Compile()).Invoke(2,5); 
        Response.Write(c); 
}

Application of Expression Tree ?
One Application I fond for the Expression tree is to make use of expression to build the dynamic linq query.
Example 1 : - Bind query to get data and sort the data dynamically. As you see in below code I build query and sorting data by passing name of the property in people object. In example Email property used to sort which can be replace by Name property.

private void BindAndSort()
    {
        List<people> people = new List<people>
            {
                new People(){ Name = "Pranay",Email="pranay@test.com",CityID=2 },
                new People(){ Name = "Heamng",Email="Hemang@test.com",CityID=1 },
                new People(){ Name = "Hiral" ,Email="Hiral@test.com",CityID=2},
                new People(){ Name = "Maitri",Email="Maitri@test.com",CityID=1 }
            };

        ParameterExpression param = Expression.Parameter(typeof(People), "People");

        Expression ex = Expression.Property(param, "Email");

        var sort= Expression.Lambda<Func<People, object>>(ex, param); 

        var sortedData =
                        (from s in people
                         select s).OrderBy<people, object>(sort.Compile()).ToList<people>();
        GridViewNotional.DataSource = sortedData;
        GridViewNotional.DataBind();
    }

Example 2 : - Build query to search data from the people entity using Email property and Email starts with "h". Over here property can be replace with the Name property.

ParameterExpression param = Expression.Parameter(typeof(People), "People");
Expression ex = Expression.Property(param, "Email");

// Get the method information for the String.StartsWith() method   
MethodInfo mi = typeof(String).
                GetMethod("StartsWith", new Type[] { typeof(String) });
MethodCallExpression startsWith = 
                Expression.Call(ex, mi, Expression.Constant("h"));

Expression<Func<People, bool>> expression =     
            Expression.Lambda<Func<People, bool>>(startsWith, param);

var searchData = (from s in people
             select s).Where(expression.Compile()).ToList();        

Both the Example 1 and Example 2 methods can be replace with the generics.

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C# generics with primary datatype

Recently me and my friend discuss about the Generics in C# after long discussion we agreed on one common definition. which is as below.
Generic is feature which is useful when you have set of types which is going to perform some set of the functions which are same but the output differ from one type to another type.
or alternatively
is way of saying "whatever the type is, I want to be able to do some common set of operation(s)." The implementation of the operation is different, but the idea is the same.
To understand this thing properly I created one console application in which we created one method which do the addition as below.

public T Add<T>(T t1, T t2)
{
        T t3 = t1 + t2;
        return t3;
}

so as per the generic definition this should work but when I complied, I got following compile time error.
Error 1 Operator '+' cannot be applied to operands of type 'T'.
First when we encounter error I think that might be I did something wrong. After sometime I realise the thing that what ever you passed as generic types its a object. So if I pass any primary data type it first get boxed automatically and than generic function take care of that. By following this fact if we apply the +,- etc. kind of arithmetic operation on the primary dataype in the generic function not going to work because its get converted in object i.e boxed.
To resolve the problem with the primary datatype in generic function I code my above function as below.

public T Add<T>(T val1, T val2)
    {
        if (val1 == null)
            throw new ArgumentNullException("val1");
        if (val2 == null)
            throw new ArgumentNullException("val2");

        object n1 = val1,
               n2 = val2;

        if (val1 is byte)
            return (T)((object)((byte)n1 + (byte)n2));
        if (val1 is short)
            return (T)((object)((short)n1 + (short)n2));
        if (val1 is int)
            return (T)((object)((int)n1 + (int)n2));
        if (val1 is long)
            return (T)((object)((long)n1 + (long)n2));
        if (val1 is float)
            return (T)((object)((int)n1 + (int)n2));
        if (val1 is double)
            return (T)((object)((double)n1 + (double)n2));

        throw new InvalidOperationException("Type " + typeof(T).ToString() + " is not supported.");

    }

As you can see in above code first I check the type of the object and if the type of object match than I perform the function that do the add operation. Basically I am unboxing parameter passed to the function.

Conclusion
In generic primary datatype treated as boxed object so when you are coding generic for primary datatype beware that you need to convert passed parameter in primary datatype i.e you need to unbox the values again.