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How to Resolve Error "CS0846: A nested array initializer is expected" in C#

The Compiler Error CS0846 is a syntax error regarding Array Initialization. The message reads: "A nested array initializer is expected".

In C#, Rectangular Arrays (multi-dimensional arrays defined with commas like int[,]) represent a strict grid structure. Unlike a flat list, the compiler requires you to group the initialization values into specific rows and columns using nested curly braces { }. If you provide a flat list of values for a multi-dimensional array, the compiler raises CS0846 because it cannot determine where one row ends and the next begins.

This guide explains how to properly format initializers for 2D and 3D arrays.

Understanding Array Dimensions​

  • 1D Array (int[]): A flat line. Initialized with one set of braces: { 1, 2, 3 }.
  • 2D Array (int[,]): A grid (Rows/Cols). Initialized with nested braces: { {1, 2}, {3, 4} }.
  • 3D Array (int[,,]): A cube. Initialized with triple nested braces.

The level of nesting in your { ... } block must strictly match the "Rank" (number of dimensions) of the array.

Scenario 1: Initializing a 2D Array (Matrix)​

This is the most common occurrence. You define a 2D array (e.g., a 2x2 grid), but you provide the data as a single continuous list.

Example of error​

public class MatrixData
{
    public void Load()
    {
        // ⛔️ Error CS0846: A nested array initializer is expected.
        // We defined a 2D array 'int[,]', but we provided a 1D list.
        // The compiler expects groups representing rows.
        int[,] grid = new int[2, 2] { 1, 2, 3, 4 };
    }
}

Solution: Group by Rows​

You must wrap each row in its own set of curly braces.

public class MatrixData
{
    public void Load()
    {
        // ✅ Correct: 
        // Outer braces = The Array
        // Inner braces = The Rows
        int[,] grid = new int[2, 2] 
        { 
            { 1, 2 }, // Row 0
            { 3, 4 }  // Row 1
        };
        
        System.Console.WriteLine(grid[1, 0]); // Output: 3
    }
}
note

Jagged Arrays ([][]) vs Rectangular ([,]): A jagged array is an "array of arrays". Its initialization syntax looks similar but behaves differently. int[][] jagged = new int[][] { new int[] { 1, 2 }, new int[] { 3 } }; CS0846 specifically applies to the comma syntax [,].

Scenario 2: Initializing a 3D Array​

As dimensions increase, the requirement for nesting increases. A 3D array represents a collection of 2D grids (pages/depth).

Example of error​

Providing only 2 levels of nesting for a 3-level array.

public class Graphics
{
    public void InitVolume()
    {
        // ⛔️ Error CS0846: A nested array initializer is expected.
        // This looks like a 2D initialization, but the type is 3D '[,,]'.
        int[,,] cube = new int[2, 2, 2] 
        { 
            { 1, 2 }, { 3, 4 }, 
            { 5, 6 }, { 7, 8 } 
        };
    }
}

Solution: Add Depth Nesting​

You need sets of braces for the "Depth", inside that for the "Height", inside that for the "Width".

public class Graphics
{
    public void InitVolume()
    {
        // ✅ Correct: 3 levels of braces for a Rank-3 array.
        int[,,] cube = new int[2, 2, 2] 
        {
            // Layer 1
            {
                { 1, 2 }, // Row 1
                { 3, 4 }  // Row 2
            },
            // Layer 2
            {
                { 5, 6 }, 
                { 7, 8 } 
            }
        };
    }
}

Conclusion​

CS0846 is a structural mismatch in your array declaration.

  1. Check the Commas: Count the commas in the type definition [,,].
    • 0 commas = 1D (No nesting needed).
    • 1 comma = 2D (Requires { { ... }, { ... } }).
    • 2 commas = 3D (Requires { { { ... } } }).
  2. Match the Braces: Ensure your initialization data has a depth of curly braces equal to the array's rank.

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