它始终是合法的，在D编程嵌套if-else语句，这意味着你可以使用一个if或else if语句在另一个if或else if语句。

语法

嵌套的if语句的语法如下：

if( boolean_expression 1)
{
   /* Executes when the boolean expression 1 is true */
   if(boolean_expression 2)
   {
      /* Executes when the boolean expression 2 is true */
   }
}

可以嵌套else if...else 以类似的方式，如嵌套的if语句。

例子：

import std.stdio;
 
int main ()
{
   /* local variable definition */
   int a = 100;
   int b = 200;
 
   /* check the boolean condition */
   if( a == 100 )
   {
       /* if condition is true then check the following */
       if( b == 200 )
       {
          /* if condition is true then print the following */
          writefln("Value of a is 100 and b is 200" );
       }
   }
   writefln("Exact value of a is : %d", a );
   writefln("Exact value of b is : %d", b );
 
   return 0;
}

当上面的代码被编译并执行，它会产生以下结果：

Value of a is 100 and b is 200
Exact value of a is : 100
Exact value of b is : 200

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