Is there a difference between using square brackets and curly braces in dictionary initializers?

Question

In dictionary initializers, there are two ways to create the dictionary with content:

new Dictionary<string, GradientSpace>
{
    ["userSpaceOnUse"] = GradientSpace.Absolute,
    ["objectBoundingBox"] = GradientSpace.Relative
})

and

new Dictionary<string, GradientSpace>
{
    {"userSpaceOnUse", GradientSpace.Absolute},
    {"objectBoundingBox", GradientSpace.Relative}
});

Is there any difference at all in these two syntaxes, or is it just a matter of preference?

Answer 1

When the source is as follows and the target framework is .NET 4.7.1:

var x = new Dictionary<string, int>
{
    ["userSpaceOnUse"] = 1,
    ["objectBoundingBox"] = 3
};

var y = new Dictionary<string, int> {
    {"userSpaceOnUse", 1},
    {"objectBoundingBox", 3}
};

This results in the following Intermediate Language (using JetBrains dotPeek):

// [18 13 - 22 15]
IL_0001: newobj       instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::.ctor()
IL_0006: dup          
IL_0007: ldstr        "userSpaceOnUse"
IL_000c: ldc.i4.1     
IL_000d: callvirt     instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::set_Item(!0/*string*/, !1/*int32*/)
IL_0012: nop          
IL_0013: dup          
IL_0014: ldstr        "objectBoundingBox"
IL_0019: ldc.i4.3     
IL_001a: callvirt     instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::set_Item(!0/*string*/, !1/*int32*/)
IL_001f: nop          
IL_0020: stloc.0      // x

// [25 13 - 28 15]
IL_0021: newobj       instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::.ctor()
IL_0026: dup          
IL_0027: ldstr        "userSpaceOnUse"
IL_002c: ldc.i4.1     
IL_002d: callvirt     instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0/*string*/, !1/*int32*/)
IL_0032: nop          
IL_0033: dup          
IL_0034: ldstr        "objectBoundingBox"
IL_0039: ldc.i4.3     
IL_003a: callvirt     instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0/*string*/, !1/*int32*/)
IL_003f: nop          
IL_0040: stloc.1      // y

The first way results in the indexer/property being set, while the second way uses the Add() method, meaning they are translated differently.

The .NET Core source of the Dictionary class is also interesting to look at in this context: https://github.com/Microsoft/referencesource/blob/master/mscorlib/system/collections/generic/dictionary.cs