A declared type is a contract for every assignment

First, build a mental model

A variable has a type even when you let the compiler infer it. C# source is compiled for .NET; a browser does not execute that source as JavaScript.

1. int stores whole numbers; bool stores a decision; string stores text. Choose the representation for the operations you need.
2. var means infer a static type from this initializer. It does not mean the type can change on the next line.
3. Two int operands use integer division: 5 / 2 is 2. Introduce an appropriate fractional operand when the contract needs a fraction.
4. decimal represents base-10 values with a finite range and precision; the m suffix selects a decimal literal. Rounding rules still need an explicit business decision.

int score = 10;
var bonus = 3; // bonus has type int
score += bonus;
int half = 5 / 2;
decimal price = 12.50m;
decimal total = price * 2;
Console.WriteLine(score);
Console.WriteLine(half);

1. Bind

   score and bonus are both int. The inferred type of bonus stays int.

2. Calculate

   score becomes 13; integer division makes half equal 2.

3. Validate

   The compiler checks type compatibility. It does not decide whether half was the intended business result.