Object vs Var vs Dynamic

Object: 
Each object in C# is derived from object type, either directly or indirectly. It is compile time variable and require boxing and unboxing for conversion and it makes it slow. You can change value type to reference type and vice versa. 

        public void CheckObject()

        {

            object test = 10;

            test = test + 10;    // Compile time error

            test = "hello";      // No error, Boxing happens here

        }

Var: 
It is compile time variable and does not require boxing and unboxing. Since Var is a compile time feature, all type checking is done at compile time only. Once Var has been initialized, you can't change type stored in it.

        public void CheckVar()

        {

            var test = 10;         // after this line test has become of integer type

            test = test + 10;    // No error

            test = "hello";        // Compile time error as test is an integer type

        }

Dynamic: 
It is run time variable and not require boxing and unboxing. You can assign and value to dynamic and also can change value type stored in same. All errors on dynamic can be discovered at run time only. We an also say that dynamic is a run time object which can hold any type of data.

        public void CheckDynamic()

        {

            dynamic test = 10;

            test = test + 10;    // No error

            test = "hello";       // No error, neither compile time nor run time

        }

So here, All these variables are handy and can be used on the basis of our needs. Var is mainly used in case of LINQ queries and dynamic is used mainly in COM interop cases. I used dynamic mainly for COM interaction and as a replacement of Reflection.