How to invoke or instantiate obj using private constructor?

Hello, i would like to know how to instantiate an object using private constructor? and what are the main benefits to use private constructor?

namespace Practice

{

public class Test

{

int a;

public Test()

{

}

private Test(int a)

{

this.a = a;

}

public class Test2

{

void method()

{

Test obj1 = new Test();

}

As the code stands the only way to instantiate a Test object using its private constructor is to use reflection. Reflection is also needed to invoke the private Test2.method() and to get the value of the private field Test.a. Here's some code to do all that:

using System;

using System.Reflection;

using Practice;

namespace Practice

{

public class Test

{

int a;

public Test()

{

}

private Test(int a)

{

this.a = a;

}

public class Test2

{

void method()

{

Type t = typeof(Test);

BindingFlags bf = BindingFlags.Instance | BindingFlags.NonPublic;

ConstructorInfo[] cia = t.GetConstructors(bf);

Test test = (Test)cia[0].Invoke(new object[]{42});

// now get the private field 'a' using reflection

FieldInfo fi = t.GetField("a", bf);

int a = (int)fi.GetValue(test);

Console.WriteLine("The value of 'a' is " + a);

}

class Program

{

static void Main()

{

Test2 test2 = new Test2();

Type t = typeof(Test2);

BindingFlags bf = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.InvokeMethod;

t.InvokeMember("method", bf, null, test2, null);

Console.ReadKey();

}

The output is :

The value of 'a' is 42

2 Replies

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Vulpes11y ago
Accepted answer
As the code stands the only way to instantiate a Test object using its private constructor is to use reflection. Reflection is also needed to invoke the private Test2.method() and to get the value of the private field Test.a. Here's some code to do all that:

using System;
using System.Reflection;
using Practice;

namespace Practice
{
public class Test
{
int a;
public Test()
{
}

private Test(int a)
{
this.a = a;
}
}

public class Test2
{
void method()
{
Type t = typeof(Test);
BindingFlags bf = BindingFlags.Instance | BindingFlags.NonPublic;
ConstructorInfo[] cia = t.GetConstructors(bf);
Test test = (Test)cia[0].Invoke(new object[]{42});

// now get the private field 'a' using reflection
FieldInfo fi = t.GetField("a", bf);
int a = (int)fi.GetValue(test);
Console.WriteLine("The value of 'a' is " + a);
}
}
}

class Program
{
static void Main()
{
Test2 test2 = new Test2();
Type t = typeof(Test2);
BindingFlags bf = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.InvokeMethod;
t.InvokeMember("method", bf, null, test2, null);
Console.ReadKey();
}
}

The output is :

The value of 'a' is 42
Bhushan Gawale11y ago
Refer this
http://msdn.microsoft.com/en-us/library/kcfb85a6.aspx